handle pending cancellation when enabling async cancellation

this is not strictly required by the standard, but without it, there
is a race condition where cancellation arriving just before async
cancellation is enabled might not be acted upon. it is impossible for
a conforming application to work around this race condition since
calling pthread_testcancel after setting async cancellation mode is
not allowed (pthread_testcancel is not specified to be
async-cancel-safe). thus the implementation should be responsible for
eliminating the race, from a quality-of-implementation standpoint.

src/thread/pthread_setcanceltype.c

@@ -6,5 +6,6 @@ int pthread_setcanceltype(int new, int *old)
 	if (new > 1U) return EINVAL;
 	if (old) *old = self->cancelasync;
 	self->cancelasync = new;
+	if (new) pthread_testcancel();
 	return 0;
 }