iptables: die quietly if parent process dies.

If we can't communicate with the parent process, he's probably died
unexpectedly; just shut down and don't bother people about it.

iptables.py

@@ -87,9 +87,12 @@ def main(port, subnets):
     # authentication as early in the startup process as possible).
     sys.stdin.readline(128)
     try:
+        debug1('iptables manager: starting transproxy.\n')
         do_it(port, subnets)
 
         sys.stdout.write('STARTED\n')
+        
+        try:
             sys.stdout.flush()
             
             # Now we wait until EOF or any other kind of exception.  We need
@@ -97,5 +100,14 @@ def main(port, subnets):
             # authentication at shutdown time - that cleanup is important!
             while sys.stdin.readline(128):
                 pass
+        except IOError:
+            # the parent process died for some reason; he's surely been loud
+            # enough, so no reason to report another error
+            return
+
     finally:
+        try:
+            debug1('iptables manager: undoing changes.\n')
+        except:
+            pass
         do_it(port, [])