base: do a sweep of setgroups() that mean to clear the supplementaries

In the future, this will be beneficial as we move the egid out of the
groups list; there's no need to track the egid explicitly

base: do a sweep of setgroups() that mean to clear the supplementaries

In the future, this will be beneficial as we move the egid out of the
groups list; there's no need to track the egid explicitly in our
supplemental groups, and doing so could become a security issue if
setgid() would not end up switching groups[0] as it does today and
we end up wanting to change our egid.

The rwhod diff is a little gratuitious, but I like patterns and
setgroups() -> setgid() -> setuid() is a lot more common than
setgid() -> setgroups() -> setuid().

This is sweep is not exhaustive and intentionally did not include
contrib/, crypto/ or tools/.

Reviewed by: olce
Differential Revision: https://reviews.freebsd.org/D51510

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