smp: Check for SIGCHLD in thread timer interrupt handler.

smp: Check for SIGCHLD in thread timer interrupt handler.

An idle thread might get a SIGCHLD, wake and run the thread timer
interrupt handler (since there is no subprocess-status-change
interrupt) and return to idle.  If it is not the io-waiter, it does
not call test-select-registry nor otherwise notice the subprocess
status change.

Funnel all status change notifications through handle-subprocess-
status-change.  It uses the last-global-tick variable to remember the
tick when threads blocking on status changes were last woken.