fix: /var/mail-state should not symlink non-existing directories (#4018)

Fixes an issue with the Getmail service, view PR thread for additional details.
- Log an error when the expected service state directory doesn't exist.
- The location `/var/lib/getmail/` doesn't seem like it should have been introduced. Drop it in favor of `/tmp/docker-mailserver/getmail`. It appears to be for storing remote mail that was retrieved if not configured to send to Dovecot like our docs advise. This location was never valid anyway (_as referenced issue covers_).

target/scripts/startup/setup.d/getmail.sh

@@ -31,6 +31,11 @@ function _setup_getmail() {
 EOF
       chmod -R 600 "${GETMAILRC}"
     fi
+
+    # Both the debug command and cron job (that runs getmail) for getmail
+    # expect this location to exist.
+    GETMAILDIR=/tmp/docker-mailserver/getmail
+    mkdir -p "${GETMAILDIR}"
   else
     _log 'debug' 'Getmail is disabled'
   fi