cred: supplementary_group_member() => group_is_supplementary()

The new name seems more immediately descriptive.

While here, constify its signature ('struct ucred' isn't modified).

While here, replace "supplemental" => "supplementary" in some comments.

No functional change (intended).

Reviewed by: mhorne, emaste
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46906