bridge: multicast: Export some parameters RFCs suggest to be tunable

bridge: multicast: Export some parameters RFCs suggest to be tunable

RFCs suggest some parameters of IGMP and MLD to be configurable by
the administrator. With this patch the following parameters are
configurable:

* robustness (default: 2)
* query_interval (default: 12500 [125s])
* query_response_interval (default: 1000 [10s])
* last_member_interval (default: 100 [1s])

Depending on the size and nature of the network topology administrators
might want to increase or decrease these parameters.

netifd will take care of configuring any other parameters which are
dependant on the ones above and set them according to the formulas
provided in the RFCs. These parameters of the bridge are
membership_interval, querier_interval, startup_query_interval,
startup_query_count and last_member_count.

RFCs allow setting three more parameters to be configurable:
startup_query_interval, startup_query_count and last_member_count.
However this patch does not export them, as they can be indirectly
tuned via the given, exported four parameters, too.

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