MFC freebsd-update: Clarify unsupported release upgrade error message

MFC freebsd-update: Clarify unsupported release upgrade error message

MFC r343122:

freebsd-update: Clarify unsupported release upgrade error message

Notify users that upgrading from -CURRENT or -STABLE is unsupported by
freebsd-update.

Also ensure --currently-running provides a correctly formatted release
(as done by -r).

Reported by:	yuri
Reviewed by:	bcran

MFC r343268:

frebsd-update: fix --currently-running after r343122

Reported by:	Brandon Schneider

MFC r343271:

freebsd-update: Allow upgrade from pre-release builds

Update r343122 to include -ALPHA, -BETA and -RC releases as
upgrade-able via freebsd-update.

Reported by:	delphij, des
Reviewed by:	delphij

MFC r343410:

freebsd-update: fix style from r343271 change

PR:		234771
Submitted by:	Gerald Aryeetey <aryeeteygerald_rogers.com>
Sponsored by:	The FreeBSD Foundation