arbiter: Handle SIGCHLD in normal/main process context

... as opposed to in signal context. This is beneficial, since it
means that we can, in a signal safe way, print messages about why
e.g. a worker stopped its execution.

The code is still not completely signal safe, since it uses a list to
push what signal was caught. This is addressed in the following commit.

gunicorn/arbiter.py

@@ -42,7 +42,7 @@ class Arbiter(object):
     # I love dynamic languages
     SIG_QUEUE = []
     SIGNALS = [getattr(signal, "SIG%s" % x)
-               for x in "HUP QUIT INT TERM TTIN TTOU USR1 USR2 WINCH".split()]
+               for x in "CHLD CLD HUP QUIT INT TERM TTIN TTOU USR1 USR2 WINCH".split()]
     SIG_NAMES = dict(
         (getattr(signal, name), name[3:].lower()) for name in dir(signal)
         if name[:3] == "SIG" and name[3] != "_"
@@ -186,7 +186,6 @@ def init_signals(self):
         # initialize all signals
         for s in self.SIGNALS:
             signal.signal(s, self.signal)
-        signal.signal(signal.SIGCHLD, self.handle_chld)
 
     def signal(self, sig, frame):
         if len(self.SIG_QUEUE) < 5:
@@ -237,10 +236,12 @@ def run(self):
                 self.pidfile.unlink()
             sys.exit(-1)
 
-    def handle_chld(self, sig, frame):
+    def handle_cld(self):
         "SIGCHLD handling"
         self.reap_workers()
-        self.wakeup()
+
+    def handle_chld(self):
+        self.handle_chld()
 
     def handle_hup(self):
         """\
@@ -393,6 +394,7 @@ def stop(self, graceful=True):
         self.kill_workers(sig)
         # wait until the graceful timeout
         while self.WORKERS and time.time() < limit:
+            self.reap_workers()
             time.sleep(0.1)
 
         self.kill_workers(signal.SIGKILL)