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.

src/firewall.py

 import errno
 import socket
 import select
+import signal
 import struct
 import compat.ssubprocess as ssubprocess
 import ssyslog
@@ -565,6 +566,11 @@ def main(port_v6, port_v4, dnsport_v6, dnsport_v4, method, udp, syslog):
     sys.stdout.write('READY %s\n' % method)
     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()