bridge: allow enabling or disabling the multicast querier independently of IGMP snooping

In larger networks, especially big batman-adv meshes, it may be desirable to
enable IGMP snooping on every bridge without enabling the multicast querier
to specifically put the querier on a well-connected node.

This patch adds a new UCI option 'multicast_querier' for bridges which allows
this. The default is still the value of the 'igmp_snooping' option to maintain
backwards compatiblity.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>