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

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.