x0vncserver: add new keysym in case we don't find a matching keycode

x0vncserver: add new keysym in case we don't find a matching keycode

We might often fail to find a matching X11 keycode when the client has
a different keyboard layout and end up with no key event. To avoid a
failure we add it as a new keysym/keycode pair so the next time a keysym
from the client that is unknown to the server is send, we will find a
match and proceed with key event. This is same behavior used in Xvnc or
x11vnc, although Xvnc has more advanced mapping from keysym to keycode.