sysutils/mkjail: New port: Fat jail creation script

mkjail can be used on FreeBSD to create new jails, keep them updated,
and upgrade to a new release.

mkjail requires the use of ZFS and will create each new jail in a
separate filesystem.

mkjail is not a jail manager. Jails are configured via /etc/jail.conf
and started/stopped via service jail start foo.

PR: 256871
Reported by: Robert Clausecker <fuz@fuz.su>