Support unix fds

[acrisci]

@michallowasrzechonek-silvair has pointed out that unix fds probably don't work.

By looking at the code I don't think it actually works...

As far as I understand, it's not enough to just marshall the descriptor value - one should use sendmsg and recvmsg on the socket in order to receive ancillary SOL_SOCKET data.

Write a test for unix fds and make them work correctly. Add any helper methods required to make this easy to use.