comsat: Use initgroups and setgid not just setuid

PR: 270404
Reviewed by: jlduran
Obtained from: NetBSD
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47828

(cherry picked from commit d4dd9e22c13896e6b5e2a6fc78dad4f8496cc14d)