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.
Import qmail-ldap 19980906
Import qmail-ldap 19981025
Import qmail-ldap 19981114
Import qmail-ldap 19990501
Import qmail-ldap 19990525
Import qmail-ldap 19990805
Import qmail-ldap 19991007
Import qmail-ldap 20000301-preview
Import qmail-ldap 20000325
Import qmail-ldap 20000401
Import qmail-ldap 20000501
Import qmail-ldap 20000601
Import qmail-ldap 20000701
Import qmail-ldap 20001201
Import qmail-ldap 20010101
Import qmail-ldap 20010201
Import qmail-ldap 20010301
Import qmail-ldap 20010501
Import qmail-ldap 20011001a