improve from-header guessing

[Dirk Hohndel]

The following two patches should address most of the concerns raised 
to my previous series. 

The first patch simply adds an interface to obtain a concatenation of
all instances of a specific header from an email.
The second patch uses that in order to get the full Received: headers.
It now looks at Envelope-to: and X-Original-To: headers, then at the
concatenated Received headers for either a "for email at add.res" clause
that matches a configured address or for a " by " clause that matches
the domain of a configured address.

What is still missing is the check if the host from which the mail was
received in this last case had a routable IP address.