firewall.py: catch SIGINT and SIGTERM too.

There were still a few conditions under some OSes that would cause
firewall.py to terminate without cleaning up the firewall settings.  'pkill
sshuttle' was one of them.  Ignore a couple more signals to further ensure a
correct cleanup.

(This only affects sshuttle --firewall, which is a subprocess of the main
sshuttle process.  The firewall is supposed to exit automatically whenever
the client exits, and so far that part seems to work reliably.)

firewall.py

@@ -471,6 +471,8 @@ def main(port, dnsport, syslog):
     # disappears; we still have to clean up.
     signal.signal(signal.SIGHUP, signal.SIG_IGN)
     signal.signal(signal.SIGPIPE, signal.SIG_IGN)
+    signal.signal(signal.SIGTERM, signal.SIG_IGN)
+    signal.signal(signal.SIGINT, signal.SIG_IGN)
 
     # ctrl-c shouldn't be passed along to me.  When the main sshuttle dies,
     # I'll die automatically.