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.

Set `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
compositor.

That allows Xwayland to accept both `-listenfd` on a socket and `-listen`
on a protocol.

Related: xorg/xserver#817
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
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