Sign encrypted mails.

- Husk now not only encrypts outgoing emails (if possible) but also adds
  a signature.
- Rework the internal handling of the signing key.
- Integrate the signing into the workflow of the daemon.
- Add a configuration option for a subject replacement.

e2e-tests/common.sh

@@ -18,7 +18,7 @@ send_test_mail() { # email_address
   chown $USER:mail /var/mail/$USER
   chmod 600 /var/mail/$USER
 
-  echo "test mail" | sendmail $1
+  echo -e "Subject: testmail\n\ntest mail" | sendmail $1
   inotifywait -qq -t 3 -e close_write /var/mail/$USER
 }