freebsd-version(1): Add -j flag to support jails

Make freebsd-version(1) support jails by adding the -j flag which takes
a jail jid or name as an argument. As with other options, -j
flags stack and display in the order requested.

Reviewed by: bcr (manpages), kevans
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25705