locale: handle day, abday, mon, abmon, am_pm keywords

All of these are defined as mandatory by POSIX.

While here, mark all non-standard ones as FreeBSD-only as
other systems (at least, GNU/Linux and illumos) do not handle
them, so we should not encourage their use.

PR: 237752
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D21490