groupmember(9): Detail which groups are considered, simplify

Approved by: re (gjb)
Reviewed by: mhorne
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40634

(cherry picked from commit 75a45ca3b34062fe793ae326ad9da614a1a06df1)
(cherry picked from commit d4cb91af0c93055740f33566c5f97f9dc1ec8e5d)