Multicast listener discovery for IPv6. Aims to be compliant with RFC 2710. No support for MLDv2.
Note: The allnodes (ff01::1, ff02::1) group is assumed be received by your netif since it must always be received for correct IPv6 operation (e.g. SLAAC). Ensure the netif filters are configured accordingly!
The netif flags also need NETIF_FLAG_MLD6 flag set to enable MLD6 on a netif ("netif->flags |= NETIF_FLAG_MLD6;").
To be called from TCPIP thread.