Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

locale: more output fixes

- 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()

With these changes, output of `locale` and `locale -k` for all keywords
specified by POSIX exactly matches the linux one.

PR: 237752
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D21599

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