When setting allow permissions for particular users or groups
there will be circumstances when that user is not known to the
host system.
In that case the output of `zfs allow <pool/dataset>`
looks similar to this:
$ sudo zfs allow tank/test
---- Permissions on tank/test ---------------------------------------
Local+Descendent permissions:
user (unknown: 1002) hold
user zfsuser receive
The fix in this commit removes ' (unknown: '+')' from the output
leaving only the uid/gid.
This allows the current parser to continue even if the uid/gid
is not known.
This situation occurs most often when moving a zpool from one system
to another that may not have the same users/groups. Simply adding
permissions to a user/group and then deleting the user/group
from the system will cause this situation to occur.
