Rearchitect From: header guessing code for replies

Rearchitect From: header guessing code for replies

We want to be able to correctly guess the best From: header to use when
replying to emails. This is what we are looking at now:
 1 is one of the users' mail addresses in the To: or Cc: header
 2 check for an Envelope-to: header
 3 check for an X-Original-To: header
 4 check for a (for <email@add.res>) clause in Received: headers
 5 check for the domain part of known email addresses in the
      'by' part of Received headers
 6 fall back to the primary email address

This patch changes the algorithm for steps 2-5 of this process. Prior to
this patch we had a first attempt to implement only step 5 - but this
broke in many email setups where mail delivery to the local machine added
additional Received: lines.
Steps 2-4 are new, step 5 now analyzes the concatenated Received: header
(this was in the previous patch) to do this analysis.

Signed-off-by: Dirk Hohndel <hohndel@infradead.org>