Added headers, stats, checksum parsing capabilities from RFC 2236 describing
IGMPv2.
IGMPv2 state machine is implemented for each condition, sending and receiving
IGMP Membership Reports and Leave Group messages with backwards compatibility
with IGMPv1 routers.
Test:
* Implemented igmp header parser and checksum calculator in header/igmp_test.go
* ipv4/igmp_test.go tests incoming and outgoing IGMP messages and pathways.
* Added unit test coverage for IGMPv2 RFC behavior + IGMPv1 backwards
compatibility in ipv4/igmp_test.go.
Fixes#4682
PiperOrigin-RevId: 343408809
Ghanan Gowripalan
Ryan Heacock