[Mimedefang] Adding support for learning our addresses

[Philip Prindeville]

Hmmm....  I was wondering if we might want to call
ioctl(...,  SIOCGIFCONF...) followed by SIOCGIFADDR to get the list
of our IP addresses... So we can do some filtering on people claiming
to be us.

I.e. if someone connects to me and says "helo 71.36.29.88" then I know
for a fact that they aren't me...

One other thing I thought about: what about detecting spammers, and
then looking up the CIDR block that their address belongs to, and adding
it to a blacklist automatically in filter_relay()?  We could set a
threshold for the number of offenses before they get added in.

That way, if the ratware ever gets more sophisticated and figures out
how we are detecting them and gets around it... it would be too late,
as they would be remembered as spammers.

-Philip