Throughout change all calls of low_priority_sleep (0) to yield ().

* miscfuncs.cc (yield): Rename from low_priority_sleep.  Remove all of the
logic which called Sleep() and just use SwitchToThread.
* miscfuncs.h (yield): Rename from low_priority_sleep.
(SLEEP_0_STAY_LOW): Delete unused define.
* shared.cc (memory_init): Move heap_init() call directly after shared memory
initialization to more closely mimic long-standing program flow.
* tty.cc (tty_list::terminate): Replace call to low_priority_sleep with Sleep.

winsup/cygwin/tty.cc

@@ -160,7 +160,7 @@ tty_list::terminate ()
 	      i = 0;
 	    }
 
-	  low_priority_sleep (200);
+	  Sleep (200);
 	}
 
       lock_ttys here ();