[RFE][6.4?] add multicast igmp snooping data insight to bridge module sysfs / brctl

Issue

The bridge module sysfs and brctl currently lack the ability to inspect the non-configuration IP multicast igmp snooping data. At a minimum there should be the ability to: list detected mcast router ports; list multicast groups with active subscribers and the associated interfaces. The ability to get the related timer data is probably useful as well.

Attached is 'bridge-migmp.stp', a systemtap script that use to get some of this data now, as an example of what we're looking at.