kill(pid, sig) before waitpid() returns -1 for sig != 0

This is a followup to a report back in 2011 about essentially the same issue:

https://cygwin.com/ml/cygwin/2011-04/msg00031.html

The same test program in that report demonstrates the issue, but with
kill sending any non-zero signal.  To reiterate, the problem here is
POSIX compliance with respect to sending signals to zombie processes.

http://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html
claims:

  Existing implementations vary on the result of a kill() with pid
  indicating an inactive process (a terminated process that has not been
  waited for by its parent). Some indicate success on such a call
  (subject to permission checking), while others give an error of
  [ESRCH].  Since the definition of process lifetime in this volume of
  POSIX.1-2008 covers inactive processes, the [ESRCH] error as described
  is inappropriate in this case. In particular, this means that an
  application cannot have a parent process check for termination of a
  particular child with kill().  (Usually this is done with the null
  signal; this can be done reliably with waitpid().)

In response to the originally issue, this was fixed *specifically* for
the case of kill(pid, 0).  But my reading of the above is that kill()
should return 0 in this case regardless of the signal (modulo
permissions, etc.).  On Linux, for example, when calling kill with pid
of a zombie process the kernel will happily deliver the signal to the
relevant task_struct; it will just never be acted on since the task
will never run again.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

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