zfs allow/unallow should work with numeric uid/gid

And that should work even (especially) if there is no matching user or
group name. This change allows to see and modify delegations for
deleted groups and users.

The change is originally by Xin Li.
illumos report: https://www.illumos.org/issues/6037
OpenZFS (ZoL) PR: https://github.com/openzfs/zfs/pull/10280

Obtained from: delphij
MFC after: 2 weeks