New realgroupmember()

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

Approved by: re (gjb)
Reviewed by: mhorne
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)
(cherry picked from commit cbe88592ab88c3940bedc3b2a651eb4556f4bbd8)