Prefer encryption for the peer if the message is encrypted or signed with the known key (#3844)

Note that if the message is encrypted, we don't check whether it's signed with an attached key
currently, otherwise a massive refactoring of the code is needed because for encrypted messages a
signature is checked and discarded first now.

test-data/message/thunderbird_signed_unencrypted.eml

@@ -0,0 +1,56 @@
+From - Thu, 15 Dec 2022 14:45:17 GMT
+X-Mozilla-Status: 0801
+X-Mozilla-Status2: 00000000
+Message-ID: <db254c95-c23e-2ad5-14db-11ad1f374dbf@example.org>
+Date: Thu, 15 Dec 2022 11:45:16 -0300
+MIME-Version: 1.0
+User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
+ Thunderbird/102.5.1
+Content-Language: en-US
+To: bob@example.net
+From: Alice <alice@example.org>
+Subject: test message 15:53
+X-Mozilla-Draft-Info: internal/draft; vcard=0; receipt=0; DSN=0; uuencode=0;
+ attachmentreminder=0; deliveryformat=0
+X-Identity-Key: id3
+Fcc: imap://alice%40example.org@in.example.org/Sent
+Content-Type: multipart/signed; micalg=pgp-sha256;
+ protocol="application/pgp-signature";
+ boundary="------------iX39J1p7DOgblwacjo0e7jX7"
+
+This is an OpenPGP/MIME signed message (RFC 4880 and 3156)
+--------------iX39J1p7DOgblwacjo0e7jX7
+Content-Type: multipart/mixed; boundary="------------WD4DG7TcI4p4lbzyM4toRaDw";
+ protected-headers="v1"
+From: Alice <alice@example.org>
+To: bob@example.net
+Message-ID: <db254c95-c23e-2ad5-14db-11ad1f374dbf@example.org>
+Subject: test message 15:53
+
+--------------WD4DG7TcI4p4lbzyM4toRaDw
+Content-Type: text/plain; charset=UTF-8; format=flowed
+Content-Transfer-Encoding: base64
+
+DQo=
+
+--------------WD4DG7TcI4p4lbzyM4toRaDw--
+
+--------------iX39J1p7DOgblwacjo0e7jX7
+Content-Type: application/pgp-signature; name="OpenPGP_signature.asc"
+Content-Description: OpenPGP digital signature
+Content-Disposition: attachment; filename="OpenPGP_signature"
+
+-----BEGIN PGP SIGNATURE-----
+
+wsD5BAABCAAjFiEEFKs/ZfwnS721+naMJfAHJFnkeuIFAmObMvwFAwAAAAAACgkQJfAHJFnkeuLM
+TgwAnAADX93HE5vXmuBcAbRN2HKIwMzBtRtUF4FNPKchffUvvhSNpHkW2jW7A4hOHNgVSDQdqIUn
++62NgkaKrT1bZqozOZNHXMECHtKBwXWTkIAVqcBdvscCztVIgGby56OPnzZ5y09BsRaqqE5AhDgN
+wGCLa6ipu5FYSF6+KzdO0GIPMY5aGRgVhtl4N01v4S3+r/Yu60MkN87nd15Eaqsrs60P9RmKJTt4
+hDie35kKvHnPzLNs8+xLfqPuO/P7ZbPQgkgCwMAMsMDRUYOv+k5c/bL3PKiOENuDpQ7dkKJ2OzSn
+nTcg8qhDf17vWe26C/QBhFiGEsrHNBQ1KW5by+cqjIUBJgXElFnPl35S5L3fn6JHZLcz6q+wQuJu
+vGT1mJuP//jLFkMHSexukFIVXzn41rWPLd05rBqMgwRcOHMIyzE9zaO1aa8MF2TirPaZ5lH9rx/y
+9DCU/d2sqbbYt8TGqj4hM3pqg5K22eq4KT1W7y8+28I5QfjZumLLrHBdYTnR
+=6JTB
+-----END PGP SIGNATURE-----
+
+--------------iX39J1p7DOgblwacjo0e7jX7--