setup a check to warn users about certain config

If I am not mistaken, the configuration I am checking for is the one we
should emit a warning about: in case junk mail is moved to the inbox and
no rewriting happens, we should check whether this is actually what the
user wants.

target/scripts/start-mailserver.sh

@@ -36,6 +36,7 @@ function _register_functions() {
   _register_check_function '_check_improper_restart'
   _register_check_function '_check_hostname'
   _register_check_function '_check_log_level'
+  _register_check_function '_check_spam_prefix'
 
   # ? >> Setup
 

target/scripts/startup/check-stack.sh

@@ -52,3 +52,12 @@ function _check_log_level() {
     LOG_LEVEL="${DEFAULT_LOG_LEVEL}"
   fi
 }
+
+function _check_spam_prefix() {
+  # This check should be independent of ENABLE_POP3 and ENABLE_IMAP
+  if _env_var_expect_zero_or_one MOVE_SPAM_TO_JUNK \
+  && [[ ${MOVE_SPAM_TO_JUNK} -eq 0 ]] \
+  && [[ -z ${SPAM_SUBJECT} ]]; then
+    _log 'warn' "'MOVE_SPAM_TO_JUNK=0' and 'SPAM_SUBJECT' is empty - make sure this is intended: spam e-mails might not be immediately recognizable in this configuration"
+  fi
+}