Use getsockopt to check if socket is AF_UNIX

[DemiMarie]

One can use getsockopt to validate that an incoming FD is, in fact, an AF_UNIX socket.

[stefano-garzarella]

@DemiMarie can you elaborate a bit more how to use getsockopt to do this kind of validation, so newcomers can use this issue as good first issue to fix.

[DemiMarie]

@stefano-garzarella Use getsockopt(SOL_SOCKET, SO_DOMAIN) to get the domain of the socket, and getsockopt(SOL_SOCKET, SO_TYPE) to get its type. It should also be possible to use getsockopt(SOL_SOCKET, SO_PROTOCOL) to get the protocol, but it will always be 0 for AF_UNIX sockets.