Update the warden to support jailed ZFS datasets. This can be
enabled by setting "allow.mount.zfs=1" on a jail. If this is
set, the jails ZFS dataset will be set to jailed mode, and
made visible to the jail. The user should also set
"enforce_statfs=0" and "allow.mount=1" on the jails flags as well,
for this to work properly.

Requested vie RedMine: #6495