groupmember(): Extract the supplementary group search in a separate function

This is in preparation for the introduction of the new realgroupmember()
function, which does the same search into supplementary groups as
groupmember().

Reviewed by: mhorne
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40640

(cherry picked from commit b725f232f3b09b4bcbc426854fe1545234c66965)

Approved by: markj (mentor)