freebsd-update(8): Add -j flag to support jails

Make freebsd-update(8) support jails by adding the -j flag which takes
a jail jid or name as an argument. This takes advantage of the recently
added -j support to freebsd-version(8) in order to get the version of
the installed userland.

Reviewed by: dteske, kevans
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25711