If no thread can immediately handle a signal, which has been sent

If no thread can immediately handle a signal, which has been sent
to the process, it is made pending at the main thread.  There it
could hang forever.  So also check the main thread for signal
delivery.  This workaround fixes hung tests in posixtestsuite.  The
proper solution would be to split pending signals for process and
threads.
input visa@; OK guenther@