conf: add 'sharePolicy' attribute to graphics element for vnc

conf: add 'sharePolicy' attribute to graphics element for vnc

 -vnc :5900,share=allow-exclusive
allows clients to ask for exclusive access which is
implemented by dropping other connections Connecting
multiple clients in parallel requires all clients asking
for a shared session (vncviewer: -shared switch)

 -vnc :5900,share=force-shared
disables exclusive client access.  Useful for shared
desktop sessions, where you don't want someone forgetting
specify -shared disconnect everybody else.

 -vnc :5900,share=ignore
completely ignores the shared flag and allows everybody
connect unconditionally