Add new port mail/ratelimit-policyd.

This rearranges a few things in the Makefile, such as reordering
of USES= and USE... options, and uses PORTDOCS for README.md.

I have also revised pkg-message.in and pkg-descr for grammar
and consistency (trailing colons) and added a "postfix reload" line.

PR: 226795
Submitted by: Miroslav Lachman