resolved: don't make monitoring optional

resolved: don't make monitoring optional

The socket is only accessible to privileged clients anyway, no need to
add another (user unfriendly) restriction via opt-in setting. let's just
allow this for privileged clients, mirroring "busctl monitor", or
"tcpdump" and similar, which all just work if you have privs.

(This does not break API, since we never did a release witht the
"Monitor" dbus property or config setting in place, i.e. with
cb456374e096f0ebe9b70d7ddd98e16a4be24ee6)