(mouse_init): Reset the button press/release info of the mouse driver.

(mouse_init): Reset the button press/release info of the mouse driver.
(mouse_preempted): New variable.
(dos_rawgetc): Don't generate mouse events if mouse is preempted.
(XMenuActivate): Preempt the mouse during menu-handling loop.
Release the current time slice while idling in the menu-handling
loop.  Leave the loop only if the user pressed, then released the
same button.  Discard all mouse events that are pending in the
event queue before exiting.