xvnc status updates by watching server output (like nx does)

[Antoine Martin]

We should probably watch the output of the Xvnc process like we do for NX.
Looks like we can detect if the session is connected or not:
vncext: Listening for VNC connections on port 16064
Means no need to test using a wait_for_socket.
Connections: accepted: 127.0.0.1::51920
Means we can remove the need for the client to set the CONNECTED status.
Connections: closed: 127.0.0.1::51920 (Clean disconnection)
As above for AVAILABLE.

Log Xvnc output to file ~/.Xvnc/:DISPLAY.log

Note: the code has only been tested with tigervnc...

[Antoine Martin]

done in r1920, added disconnection detection in r1925