mcastmngr: If L2 snooping is configured, enable querier.

If mixed snooping / proxying is configured the querier shall not be enabled.

For traffic to be forwarded based on the snooping of IGMP/MLD we also need to enable querier functionality:

Sets whether the bridge actively runs a multicast querier or not. When a bridge receives a 'multicast host membership' query from another network host, that host is tracked based on the time that the query was received plus the multicast query interval time. If the bridge later attempts to forward traffic for that multicast membership, or is communicating with a querying multicast router, this timer confirms the validity of the querier. If valid, the multicast traffic is delivered via the bridge’s existing multicast membership table; if no longer valid, the traffic is sent via all bridge ports. Broadcast domains with, or expecting, multicast memberships should run at least one multicast querier for improved performance.