MFC r352138, r352214, r352216:
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.
- make abday, day, abmon, mon, am_pm output quoting match linux
- workaround localeconv() issue for mon_grouping and grouping (PR172215)
- for other values not available in default locale, output -1 instead of 127 (CHAR_MAX) as returned by localeconv()
PR: 237752