Improve support for Elixir, Mix and Rebar3

This PR includes many commits related to fixing and improving compilation with newer Rebar3, Erlang and Elixir.

The commit messages should be clear enough. Here I provide a general explanation of the major changes, hopefully ready for inclusion in the release notes:

Summary of compilation methods

When compiling ejabberd from source code, you may have noticed there are a lot of possibilities. Let's take an overview before digging in the new improvements:

Tools to manage the dependencies and compilation:
- Rebar: it is nowadays very obsolete, but still does the job of compiling ejabberd
- Rebar3: the successor of Rebar, with many improvements and plugins, supports hex.pm and elixir compilation
- Mix: included with the Elixir programming language, supports hex.pm, and erlang compilation
Installation methods:
- make install: copies the files to the system
- make prod: prepares a self-contained OTP production release in _build/prod/, and generates a tar.gz file. This was previously named make rel
- make dev: prepares quickly an OTP development release in _build/dev/
- make relive: prepares the barely minimum in _build/relive/ to run ejabberd and starts it
Start scripts and alternatives:
- ejabberdctl with erlang shell: start/foreground/live
- ejabberdctl with elixir shell: iexlive
- ejabberd console/start (this script is generated by rebar3 or mix, and does not support ejabberdctl configurable options)

For example:

the CI dynamic tests use rebar3, and Runtime tries to test all the possible combinations
ejabberd binary installers are built using: mix + make prod
container images are built using: mix + make prod too, and started with ejabberdctl foreground

Several combinations didn't work correctly until now and have been fixed, for example:

mix + make relive
mix + make prod/dev + ejabberdctl iexlive
mix + make install + ejabberdctl start/foregorund/live
any method + make uninstall
rebar + make prod with Erlang 26

Use Mix or Rebar3 by default instead of Rebar to compile ejabberd

ejabberd uses Rebar to manage dependencies and compilation since ejabberd 13.10 4d8f7706240a1603468968f47fc7b150b788d62f. However, that tool is obsolete and unmaintained since years ago, because there is a complete replacement:

Rebar3 is supported by ejabberd since 20.12 0fc1aea379924b6f83f274f173d0bbd163cae1c2. Among other benefits, this allows to download dependencies from hex.pm and cache them in your system instead of downloading them from git every time, and allows to compile Elixir files and Elixir dependencies.

In fact, ejabberd can be compiled using mix (a tool included with the Elixir programming language) since ejabberd 15.04 ea8db9967fbfe53f581c3ae721657d9e6f919864 (with improvements in ejabberd 21.07 4c5641a6489d0669b4220b5ac759a4e1271af3b5)

For those reasons, the tool selection performed by ./configure will now be:

Use the program specified in option: --with-rebar=/path/to/bin
If none is specified, use the system mix
If Elixir not found, use the system rebar3
If Rebar3 not found, use the rebar3 binary included with ejabberd

Removed Elixir support in Rebar

Support for Elixir 1.1 was added as a dependency in ejabberd 15.02 01e1f677c72b923251f7021bc024319ff129d42d. This allowed to compile Elixir files. But since Elixir 1.4.5 (released Jun 22, 2017) it isn't possible to get Elixir as a dependency... it's nowadays a standalone program. For that reason, support to download old Elixir 1.4.4 as a dependency has been removed.

When Elixir support is required, better simply install Elixir and use mix as build tool:

./configure --with-rebar=mix

Or install Elixir and use the experimental Rebar3 support to compile Elixir files and dependencies:

./configure --with-rebar=rebar3 --enable-elixir

Added Elixir support in Rebar3

It is now possible to compile ejabberd using Rebar3 and support Elixir compilation. This compiles the Elixir files included in ejabberd's lib/ path. There's also support to get dependencies written in Elixir, and it's possible to build OTP releases including Elixir support.

It is necessary to have Elixir installed in the system, and configure the compilation using --enable-elixir. For example:

apt-get install erlang erlang-dev elixir
git clone https://github.com/processone/ejabberd.git ejabberd
cd ejabberd
./autogen.sh
./configure --with-rebar=rebar3 --enable-elixir
make
make dev
_build/dev/rel/ejabberd/bin/ejabberdctl iexlive

Elixir versions supported

Elixir 1.10.3 is the minimum supported, but:

Elixir 1.10.3 or higher is required to build an OTP release with make prod or make dev
Elixir 1.11.4 or higher is required to build an OTP release if using Erlang/OTP 24 or higher
Elixir 1.11.0 or higher is required to use make relive
Elixir 1.13.4 with Erlang/OTP 23.0 are the lowest versions tested by Runtime

For all those reasons, if you want to use Elixir, it is highly recommended to use Elixir 1.13.4 or higher with Erlang/OTP 23.0 or higher.

Rebar and Rebar3 binaries updated

The ejabberd source code includes the rebar and rebar3 binaries, and you can use them to compile ejabberd in case they are not installed in your operating system. Those binaries have been updated to support Erlang/OTP from version 23 up to 26.

If you are using Erlang 22.3 or older, you can download the old binaries from ejabberd 21.12, available at:

`make rel` is renamed to `make prod`

When ejabberd started to use Rebar2 build tool, that tool could create an OTP release, and the target in Makefile.in was conveniently named make rel.

However, newer tools like Rebar3 and Elixir's Mix support creating different types of releases: production, development, ... In this sense, our make rel target is nowadays more properly named make prod.

For backwards compatibility, make rel redirects to make prod.

New `make install-rel` and `make uninstall-rel`

This is an alternative method to install ejabberd in the system, based in the OTP release process. It should produce exactly the same results than the existing make install.

The benefits of make install-rel over the existing method:

this uses OTP release code from rebar/rebar3/mix, and consequently requires less code in our Makefile.in
make uninstall-rel correctly deletes all the library files

This is still experimental, and it would be great if you are able to test it and report any problem; eventually this method could replace the existing one.

Just for curiosity:

ejabberd 13.03-beta1 got support for make uninstall was added
ejabberd 13.10 introduced Rebar build tool and code got more modular
ejabberd 15.10 started to use the OTP directory structure for 'make install', and this broke make uninstall

processone / ejabberd