mcastmngr: Set the "igmp" as the default protocol for mcast proxy/snooping config
For example, in the current LUCI iptv page, option "proto" is not provided, and IGMP is assumed.
(cherry picked from commit 8b399532)
For example, in the current LUCI iptv page, option "proto" is not provided, and IGMP is assumed.
(cherry picked from commit 8b399532)