cdefs(9): Start to document what sys/cdefs.h does

Start to document all the macros and such used to implment our
portability layer. It also describes what compilation environments
we support, what compilers we support and some of the details.

This is round one. All the macros, etc are in here, but some need
descriptions.

These macros, while FreeBSD internal only, do need documentation
on when/where to use them.

This man page likely needs better organization. While better
than sys/cdefs.h, the underlying chaos in that file was hard
to leave completely behind. Suggestions welcome.

Feedback by: mhorne, Graham Perrin, Alexander Ziaee
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1313