igmp: T5736: migrate "protocols igmp" to "protocols pim"

IGMP and PIM are two different but related things.

FRR has both combined in pimd. As we use get_config_dict() and FRR reload it
is better to have both centrally stored under the same CLI node (as FRR does,
too) to just "fire and forget" the commit to the daemon.

"set protocols igmp interface eth1" -> "set protocols pim interface eth1 igmp"