coredns -- use coremain() to have single binary

[hmage]

As described here -- https://github.com/coredns/coredns/commit/9d87b953a0c6b83e214c4152a9c38082ea83246d

This solves many numerous development problems, avoids having to handle separate coredns process and lets us compile a Windows binary since there will be no longer a need for POSIX/UNIX signalling to control coredns reload and lifecycle.

Providing a copy of the commit message here just in case:

---

Enhancement of external plugin enabling (#1392)

This fix is an enhancement of external plugin enabling.
Previously, it was already able to build a customerized
coredns with plugins enabled selectively, without changing
coredns source code. However, all default plugins are
actually bundled because of the import rule:

"github.com/coredns/coredns/coremain"

The issue is best described with the following:

root@localhost:/go/src/github.com/coredns/coredns/sample# cat sample.go package main

import ( _ "github.com/coredns/forward"

    "github.com/coredns/coredns/coremain"
    "github.com/coredns/coredns/core/dnsserver"

)

var directives = []string{ "forward", "startup", "shutdown", }

func init() { dnsserver.Directives = directives }

func main() { coremain.Run() } root@localhost:/go/src/github.com/coredns/coredns/sample# root@localhost:/go/src/github.com/coredns/coredns/sample# go build -v sample.go root@localhost:/go/src/github.com/coredns/coredns/sample# ./sample -plugins root@localhost:/go/src/github.com/coredns/coredns/sample# ./sample -plugins Server types: dns

Caddyfile loaders: flag default

Other plugins: dns.auto dns.autopath dns.bind dns.cache dns.chaos dns.debug dns.dnssec dns.dnstap dns.erratic dns.errors dns.etcd dns.federation dns.file dns.forward dns.health dns.hosts dns.kubernetes dns.loadbalance dns.log dns.nsid dns.pprof dns.prometheus dns.proxy dns.reverse dns.rewrite dns.root dns.route53 dns.secondary dns.template ....

This fix moves zplugins.go to a different package/directory so that
it is possible to "only import plugins as needed".

The following is the new output after this fix:

root@localhost:/go/src/github.com/coredns/coredns/sample# ./sample -plugins Server types: dns

Caddyfile loaders: flag default

Other plugins: dns.forward dns.prometheus shutdown startup

root@localhost:/go/src/github.com/coredns/coredns/sample#

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

[hmage]

Implemented in 3d01c3512e9cd0a5d267ff54addb82b55f3bdc17