firewall: catch SIGHUP and SIGPIPE.

Not sure if this will fix anything, but it might stop the problem reported
on some MacOS versions where the firewall doesn't get cleaned up correctly.

firewall.py

-import re, errno, socket, select, struct
+import re, errno, socket, select, signal, struct
 import compat.ssubprocess as ssubprocess
 import helpers, ssyslog
 from helpers import *
@@ -398,6 +398,11 @@ def main(port, dnsport, syslog):
     sys.stdout.write('READY\n')
     sys.stdout.flush()
 
+    # don't disappear if our controlling terminal or stdout/stderr
+    # disappears; we still have to clean up.
+    signal.signal(signal.SIGHUP, signal.SIG_IGN)
+    signal.signal(signal.SIGPIPE, signal.SIG_IGN)
+
     # ctrl-c shouldn't be passed along to me.  When the main sshuttle dies,
     # I'll die automatically.
     os.setsid()