Makes the xwayland startup process two phased.
The first phase just initialises the X11 sockets.
The second phase starts the Xwayland server itself.
When starting xwayland lazily the second phase will be postponed until
a client has connected to the X11 socket.
Changes in behaviour:
The DISPLAY environment is now set immediately after the X11 sockets
are created.
When the Xwayland server is killed or crashes, the sockets will not be
recreated, but reused.
Fixes#849: Start up Xwayland lazily
c80bf1591e