gsocket: fix g_socket_details_from_fd() on Solaris

gsocket: fix g_socket_details_from_fd() on Solaris

On Solaris, getsockname() on an unconnected socket gives an addrlen of
0 and doesn't set the sockaddr. So use the SO_DOMAIN sockopt to find
the socket family in that case. (SO_DOMAIN doesn't exist everywhere,
so we can't use it unconditionally. Also, we have to only use it if
getsockname() fails, since SO_DOMAIN returns a bogus value for
accept()ed sockets on both Linux and Solaris...)