Changes in preparation for release

I recommend reviewing this PR commit by commit.

Changes

Nicer error messages when the NICE_ERR_MSG option is activated.
Support for new #include_shell directive to allow the inclusion of shell scripts with scripts generated by pnut-sh.
pnut-sh can now compile C files without main, making it possible to use generated shell scripts as libraries.
Added some bindings for commonly used commands in posix-utils.sh to be included using #include_shell.
Restore the set -u option.
Add shebang to start of generated scripts.
Add license: Simplified BSD 2-clause.
Add make install command to install pnut and pnut.sh.
Improve README.md

And a few bug fixes:

let and endlet are no longer included if local variables are not used.
Functions with empty bodies are no longer ignored (they were considered forward declarations).
Remove redundant arithmetic expansion when inlining putchar calls.