Add API for joining/leaving multicast groups

[sdroege]

See summary, ideally this should also allow defining which interface should be used

Thanks for working on this library :)

[lpeterse]

I have no experience with that specific topic, but I think it deserves its place in this library as this is a very general mechanism.
I'll leave this open for later or until someone else volunteers to implement this.

[lpeterse]

According to @illdefined this is done via the following sockopts:

• IP_ADD_MEMBERSHIP (IPPROTO_IP)
• IP_DROP_MEMBERSHIP
• IPV6_ADD_MEMBERSHIP, IPV6_DROP_MEMBERSHIP (IPPROTO_IPV6)