os: Allow for partial connection with `NoListenAll`
Xwayland can optionally be socket-activated by the Wayland compositor, in which case it would use only the sockets provided by the compositor.
However, that prevents other transport protocol from working, because when it's given a socket from the Wayland compositor, it would disable all other connections and rely solely on the given socket.
partial to a non-zero value initially to hint the XTrans function
MakeAllCOTSServerListeners that it's okay if the local connection
fails, being already handled by the socket provided by the Wayland
That allows Xwayland to accept both
-listenfd on a socket and
on a protocol.