The DNS manager reacts to NM_DEVICE_IP4_CONFIG_CHANGED events, which
are generated when there is a relevant change to an IP4 configuration.

Until now, changes to the mdns,llmnr properties were not
considered relevant (and neither minor, this is already a bug).

Promote them to relevant so that the DNS manager is notified and will
rewrite the DNS configuration when one of this properties changes.

Note that the DNS priority should be considered relevant and added
into the checksum as well, but is a problem right now because in the
DNS manager we rely on the fact that an empty configuration (i.e. just
created) has a zero checksum. This is needed to avoid rewriting
resolv.conf when there is no change. The DNS priority initial value
depends on the connection type (VPN or not), so it's a bit difficult
to add it to checksum maintaining the assumption of checksum(empty)=0.
This should be improved in the future.