arbiter: Reinstall SIGCHLD as required by some UNIXes

According to the python signal documentation[1], SIGCHLD is handled
differently from other signals. Specifically, if the underlying
implementation resets the SIGCHLD signal handler, then python won't
reinstall it (as it does for other signals).

This behavior doesn't seem to exist for neither Linux nor Mac, but
perhaps one could argue that it's good practise anyway.

[1] https://docs.python.org/3/library/signal.html

gunicorn/arbiter.py

@@ -201,6 +201,10 @@ def signal(self, sig, frame):
         """ Note: Signal context! No logging or memory allocations allowed. """
         self.write_message_to_pipe(sig)
 
+        # Some UNIXes require SIGCHLD to be reinstalled, see python signal docs
+        if sig == signal.SIGCHLD:
+            signal.signal(sig, self.signal)
+
     def run(self):
         "Main master loop."
         self.start()