Check DKIM Authentication-Results (#3583)

Fix #3507

Note that this is not intended for a release at this point! We first have to test whether it runs stable enough. If we want to make a release while we are not confident enough in authres-checking, then we have to disable it.

BTW, most of the 3000 new lines are in `test_data/messages/dkimchecks...`, not the actual code

da3a4b94 adds the results to the Message info. It currently does this by adding them to `hop_info`. Maybe we should rename `hop_info` to `extra_info` or something; this has the disadvantage that we can't rename the sql column name though.

Follow-ups for this could be:
- In `update_authservid_candidates()`: Implement the rest of the algorithm @hpk42 and me thought about. What's missing is remembering how sure we are that these are the right authserv-ids. Esp., when receiving a message sent from another account at the same domain, we can be quite sure that the authserv-ids in there are the ones of our email server. This will make authres-checking work with buzon.uy, disroot.org, yandex.ru, mailo.com, and riseup.net.
- Think about how we present this to the user - e.g. currently the only change is that we don't accept key changes, which will mean that the small lock on the message is not shown.
- And it will mean that we can fully enable AEAP, after revisiting the security implications of this, and assuming everyone (esp. @link2xt who pointed out the problems in the first place) feels comfortable with it.

test-data/message/dkimchecks-2022-09-28/alice@buzon.uy/alice@gmail.com

@@ -0,0 +1,5 @@
+Authentication-Results: mail.buzon.uy;
+	dkim=pass (2048-bit key; unprotected) header.d=gmail.com header.i=@gmail.com header.b="Ngf1X5eN";
+	dkim-atps=neutral
+From: <alice@gmail.com>
+To: <alice@buzon.uy>