freebsd-update: Error for -b basedir without UNAME_r set

freebsd-update sets the currently running release from UNAME -r, which
can be overridden via the --currently-running commandline option (or by
setting UNAME_r in the environment). This may be invalid if -b is used
to specify a basedir other than /, so error out if -b is specified
without setting the currently running version.

PR: 283229
Reviewed by: cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48016