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.