vnc: implement shared flag handling.

vnc: implement shared flag handling.

VNC clients send a shared flag in the client init message.  Up to now
qemu completely ignores this.  This patch implements shared flag
handling.  It comes with three policies:  By default qemu behaves as one
would expect:  Asking for a exclusive access grants exclusive access to
the client connecting.  There is also a desktop sharing mode which
disallows exclusive connects (so one forgetting -shared wouldn't drop
everybody else) and a compatibility mode which mimics the traditional
(but non-conforming) qemu behavior.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>