cred: Constify signature of groupmember() and realgroupmember()
Reviewed by emaste
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D47343
(cherry picked from commit b15110fb0dc6f0480482616f22bda3a2a19fbd80)
Approved by: markj (mentor)