bridge: Allow setting multicast_router option

bridge: Allow setting multicast_router option

The multicast_router option of a bridge allows to control the forwarding
behaviour of multicast packets independant of the listener state:

* 0: Only forward if specific listener is present
* 1 (default): Forward if specific listener or a multicast router
  was detected (currently only learned via query messages, no MRD
  support yet)
* 2: Always forward any multicast traffic on this port

Since MRD is not mandated you might end up with silent multicast routers
(e.g. if your link has more than one multicast router; only one can
become the selected, "noisy" querier). Here you might need a manual
configuration option like the "multicast_router" option.

Other scenarios where this can be useful are for instance:
* Segmentation of IGMP/MLD domains together with ebtables
* Dedicated bridge port for monitoring/debugging purposes

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>