comsat: move uid/gid setting earlier

It's good to reduce privilege as early as possible.

Suggested by: jlduran
Reviewed by: jlduran
Obtained from: NetBSD
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47869

(cherry picked from commit 91629228e3df14997df12ffc6e7be6b9964e5463)