mdo(1): Use setcred() to change credentials

As this is the only system call that MAC/do currently supports, and the
only one that really can be for transitions involving simultaneous
changes of user and group IDs.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47621

(cherry picked from commit e395e354823b690ba19ecc8e3688bacec6f67ad3)