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/thread.cc

@@ -456,7 +456,7 @@ pthread::create (void *(*func) (void *), pthread_attr *newattr,
     {
       postcreate ();
       while (!cygtls)
-	low_priority_sleep (0);
+	yield ();
     }
   retval = magic;
   mutex.unlock ();