bus-proxy: never pass on unmatched broadcasts

The lovely libvirtd goes into crazy mode if it receives broadcasts that
it didn't subscribe to. With bus-proxyd, this might happen in 2 cases:

    1) The kernel passes us an unmatched signal due to a false-positive
       bloom-match.

    2) We generate NameOwnerChanged/NameAcquired/NameLost locally even
       though the peer didn't subscribe to it.

dbus-daemon is reliable in what signals it passes on. So make sure we
follow that style. Never ever send a signal to a local peer if it doesn't
match an installed filter of that peer.