plowlist: add new module list variable MODULE_xxx_LIST_HAS_SUBFOLDERS

This saves some code in modules:

<code>
test "$2" && log_debug 'recursive folder does not exist in hoster.com'
</code>

or

<code>
if test "$2"; then
  log_error 'Recursive flag has no sense here, abort'
  return $ERR_BAD_COMMAND_LINE
fi
</code>

Exiting with an error was excessive, you'll now get a log_notice message.