groupmember(), realgroupmember(): Return a bool instead of an int

Requested by: mhorne
Reviewed by: mhorne
MFC after: 2 weeks
MFC to: stable/14 releng/14.0
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40958
Differential Revision: https://reviews.freebsd.org/D40959

(cherry picked from commit ffd3ef8ee0253ffaf214cf711251d112f6a2bcf6)
(cherry picked from commit 845b7c80887ac84c82ee776836ef86d68ea71c94)