timeslice' wait.
(setup_handler): Revamp YA to attempt to deliver signal multiple times on
failure rather than punting to wait_sig.
* miscfuncs.cc (low_priority_sleep): Accept INFINITE sleep to mean "don't reset
the priority after setting it to idle". Return previous priority.
* winsup.h (SLEEP_0_STAY_LOW): Define.
(low_priority_sleep): Return an int.
* sigproc.h: Define WAIT_SIG_PRIORITY here.
* sigproc.cc: Remove WAIT_SIG_PRIORITY definition.