kern: add a new ucred flag for groups having been set

Now that we can legitimately have ngroups == 0 as a result of calling
crsetgroups(), set a flag when we've set groups for the sake of sanity
checking usage of crextend(). While it's true this flag will only
really be used under INVARIANTS, it's only the second flag bit that
we're adding in 16 years.

Reviewed by: olce
Differential Revision: https://reviews.freebsd.org/D51646