iptables: more resilient startup/cleanup.

Now the sudo iptables subprocess persists for the entire life of sshuttle.
The benefits of this are:

- no need to authenticate again at shutdown (failure of which could cause us
  to not clean up iptables)

- if the main process dies unexpectedly, iptables still gets cleaned up

- the password prompt can happen *before* starting the ssh/server process,
  which means it'll stand out and the password prompt won't be overwritten.

main.py

@@ -83,5 +83,6 @@ except Fatal, e:
     log('fatal: %s\n' % e)
     sys.exit(99)
 except KeyboardInterrupt:
-    log('\nKeyboard interrupt: exiting.\n')
+    log('\n')
+    log('Keyboard interrupt: exiting.\n')
     sys.exit(1)