Rewrite language_negotiation_include to support form_state and use it in language.admin.inc

Coming from #1468930: Clean up and move the code for the negotiation functionality from locale module to language module →

+++ b/core/modules/language/language.admin.inc
@@ -436,3 +442,375 @@ function language_admin_predefined_list() {
+function language_negotiation_configure_url_form($form, &$form_state) {
...
+  language_negotiation_include();
None of these form constructor functions should include support files directly. Form functions always have to use form_load_include().

This bug existed before already, so no need to fix it in this monster. However, please create a follow-up issue for that. (this fix can/should be backported)

+++ b/core/modules/language/language.module
@@ -211,3 +259,281 @@ function language_css_alter(&$css) {
+function language_negotiation_include() {
...
+  include_once DRUPAL_ROOT . '/' . drupal_get_path('module', 'language') . '/language.negotiation.inc';
Why isn't this using module_load_include()?

Speaking of, you could add an optional &$form_state = NULL parameter to this helper function, so you can also call this function from form constructors. However, we can do that in a follow-up issue (as mentioned above).