Adds tarpit­ting to qmail-smt­pd, which is al­most manda­to­ry for a mod­ern large SMTP serv­er.

Too bad it's not pro­gres­sive tarpit­ting, but that's easy to fix :-)

Now all I need is a patch to make qmail-smt­pd check lo­cal rcp­t's va­lid­i­ty and I'm com­plete­ly trou­ble-free with it.