cred: groupmember() and co.: Sanity check cred's groups (INVARIANTS)

Leverage the normalization check functions introduced in the previous
commit in all public-facing groups search functions to catch programming
errors early.

Approved by: markj (mentor)
MFC after: 3 days

(cherry picked from commit 634675067867090e538b08e62ff9b14d3ffae5a3)

Approved by: markj (mentor)