wrap comment
add link to https://www.drupal.org/node/2309531
add link to follow-up https://www.drupal.org/node/2310173
add menu_link_content_help
START PART 5: https://www.drupal.org/node/2301319
apply patch https://www.drupal.org/files/issues/menu-2256521-remove-dead-code.patch
and resolve conflicts with 8.x
remove unused constant MENU_MAX_DEPTH