cred: New crsetgroups_fallback()

Similar to crsetgroups(), but allows an empty group array in input,
treating it like a one-element array containing the passed fallback
group.

Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46917

(cherry picked from commit d4e0d4d92e010b74404bddc87c1a1e8dbcaa25dd)

Approved by: markj (mentor)