FreeBSD/src 0d5c524 (r343122)usr.sbin/freebsd-update freebsd-update.sh

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).

PR:             234771
Submitted by:   Gerald Aryeetey <aryeeteygerald_rogers.com>
Reported by:    yuri
Reviewed by:    bcran
MFC after:      1 month
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D18803
DeltaFile
+22-1usr.sbin/freebsd-update/freebsd-update.sh
+22-11 files

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