gtk: Vanquish spinning gtk-thread evil (again).

gtk: Vanquish spinning gtk-thread evil (again).

tests/runtime/test-gtk spins like crazy again.  With a gtk-thread
always ready to run and no thread waiting for io, wait-for-io is never
called and maybe-signal-io-thread-events never blocks.  Unfortunately
test-select-registry always returned INTERRUPT for non-blocking
selects with an empty registry.  It needs to return PROCESS-STATUS-
CHANGE when there is a pending process status change so that handle-
subprocess-status-change runs.  Thus OS_pause got a new argument:
blockp.  In uxio.c it can call OS_process_any_status_change and
return PROCESS-STATUS-CHANGE when necessary.