New realgroupmember()

Like groupmember(), but taking into account the real group instead of
the effective group. Leverages the new supplementary_group_member()
function.

Reviewed by: mhorne
MFC after: 2 weeks
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40641
Differential Revision: https://reviews.freebsd.org/D40643

(cherry picked from commit 2a2bfa6ad92e9c82dcc55733ad2fd58fd2ea7559)
(cherry picked from commit 5d9f38405a10fdcd9fc108c940dcf2642e9f1833)