MINOR: sockpair: implement the .rx_listening function

MINOR: sockpair: implement the .rx_listening function

For socket pairs we don't rely on a real listening socket but we need to
have a properly connected UNIX stream socket. This is what the new
sockpair_accept_conn() tries to report. Some corner cases like half
shutdown will still not be detected but that should be sufficient for
most cases we really care about.