[ISSUE] mrep: Use first instead of last delivered-to to set from address

[xelxebar <xelxebar@users.noreply.github.com>]

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New issue by xelxebar on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/233

Description:
## Overview

Commit e1e5bfe5 introduces a regression against 282de650. In cases where there are multiple `Delivered-To` headers, it is most likely that we want `mrep` to populate the sender address from the most public-facing `Delivered-To`, *i.e.* the first one; however, at the moment we are setting it to the address in the final (*i.e.* topmost and most internal-facing) one.

## Why?

We have a posix setup that aliases multiple addresses into a virtual mailbox---*e.g.* all mails going to `mars.*@example.com` get glommed into `mars@example.com`. The problem is that postfix aliases end up adding an additional `Delivered-To` header for each of these alias redirections. This is necessary to detect alias loops and related problems.

## Patch points

This proposed change boils down to munging [mcom:377](https://github.com/leahneukirchen/mblaze/blob/master/mcom#L377). I believe we just need to reverse the order of headers passed to `maddr` and then `tail -n 1` instead of `head -n 1`. However, it appears that `maddr` doesn't currently have a `-M` switch like `mhdr`, so maybe a patch there is needed as well?

## Comments

Thoughts? Does the above sound reasonable?