SPF implementation for qmail

SPF is something like a DNS based "reverse MX" system to designate
permitted senders for mails depending on the domain name. The goal is to
disallow sender address forgery.

The patch is entirely written using the qmail string and DNS functions, and
thus completely integrated into qmail without external dependencies.

It can check incoming mails inside the SMTP daemon, add Received-SPF lines
and optionally block undesired transfers. The check is performed at the
envelope level.