xwayland: add (fake) device grab support

Add a new command line option "-host-grab" to disable the keyboard
shortcuts and confine the pointer on the host so that Xwayland can
receive all keyboard events.

This is useful when running a complete desktop environment within
Xwayland rootful.

Use [CTRL]+[SHIFT] to release the keyboard and pointer.

This option is not compatible with rootless mode.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>