comsat: Improve use of setuid()

Just return from jkfprintf if either (a) user lookup fails (that is,
getpwnam fails) or (b) setuid() to the user's uid fails. If comsat is
invoked from inetd using the default of tty:tty we will now return due
to setuid() failing rather than fopen() failing.

PR: 270404
Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47823