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/bin/debug-getmail

@@ -7,13 +7,7 @@ source /usr/local/bin/setup.d/getmail.sh
 
 _setup_getmail
 
-if [[ -d /var/lib/getmail ]]; then
-  GETMAILDIR=/var/lib/getmail
-else
-  mkdir -p /tmp/docker-mailserver/getmail
-  GETMAILDIR=/tmp/docker-mailserver/getmail
-fi
-
+GETMAILDIR=/tmp/docker-mailserver/getmail
 for FILE in /etc/getmailrc.d/getmailrc*; do
   getmail --getmaildir "${GETMAILDIR}" --rcfile "${FILE}" --dump | tail -n +6
 done