Eio.Net: IPv6 multicast support

[adatario]

IPv6 offers support for sending packets to multiple recipients. This is done via multicast groups. A list of multicast groups is maintained by the IANA.

In Unix a socket that is used for multicast must be joined to a group via a setsockopt call:

setsockopt(sd, IPPROTO_IPV6, IPV6_JOIN_GROUP, <group>, ...)

It would be nice for Eio to expose an API for doing IPv6 multicast.

See also:

• ocaml-ipv6-multicast: OCaml Unix bindings for IPv6 multicast
• Discussion on UDP support (https://github.com/ocaml-multicore/eio/pull/171)
• A CoAP library that could use multicast IPv6 for service discover

[avsm]

+1 for multicast support; we need to do this systematically for IPv4 sockets as well.