Test the interaction of signals with multiple posix threads.

Signal SIGUSR1 is used for thread coordination, SIGUSR2 to test
signal delivery.  First SIGUSR1 and SIGUSR2 get blocked.  Then a
given number of threads are created, the main thread is also counted
as a thread.

Signal SIGUSR2 is send to the process with kill(2), but it is not
delivered yet as it is blocked in all threads.  Meanwhile the threads
wait for SIGUSR1 in sigsuspend(2).  This enforces that SIGUSR2 is
marked as pending at the process.  To continue, SIGUSR1 is sent to
all threads and they wake up.  Only one thread is configured to
handle SIGUSR2, this one unblocks it with pthread_sigmask(3).  The
signal should be delivered immediately, the handler records it.

The test is considered successful if the thread that unblocks SIGUSR2
actually handles it.

To test different race conditions, sleeps can be inserted.  If the
kill(2) is delayed, SIGUSR2 hits the threads when they are in
sigsuspend(2).  If the sleep is before sigsuspend(2), the threads
are in nanosleep(2).  The unblocking pthread_sigmask(3) can be
delayed so that the other threads have been joined already.

It is also possible to avoid blocking the signals and check which
handler catches it.  Alternatively sigwait(3) can be used to test
signal reception.