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/fhandler_proc.cc

@@ -616,7 +616,7 @@ format_proc_cpuinfo (void *, char *&destbuf)
 	debug_printf ("SetThreadAffinityMask failed %E");
       // I'm not sure whether the thread changes processor immediately
       // and I'm not sure whether this function will cause the thread to be rescheduled
-      low_priority_sleep (0);
+      yield ();
 
       bool has_cpuid = false;