Clean up w32 timer thread code in the hope of solving bug #12832.

Clean up w32 timer thread code in the hope of solving bug #12832.

 src/w32proc.c (timer_loop): Make sure SuspendThread and ResumeThread
 use the same value of thread handle.
 (start_timer_thread): If the timer thread exited (due to error),
 clean up by closing the two handles it used.  Duplicate the caller
 thread's handle here, so it gets duplicated only once, when
 launching the timer thread.  Set priority of the timer thread, not
 the caller thread.
 (getitimer): Don't duplicate the caller thread's handle here.