Per-auth-backend field locks support. Merged from MOODLE_15_STABLE

* Extended set_config()
 * Implemented get_config() which takes over $CFG loading in setup.php
 * admin/auth.php has special handling if post vars starting in pluginconfig_
 * admin/auth.php print_auth_lock_options() prints a form fragment -- being called from most plugins now
 *  user/edit.php follows the new convention when locking down fields, both javascript UI and on POST.
 * admin/auth: More solid checking for auth GET/POST var.
 * admin/auth: print_auth_lock_options() now handles user field mapping options for LDAP and similar modules
 * admin/auth: user mapping options have moved to config_plugins table
 * auth/ldap module has migrated to using new field mapping vars -- simplified config.html a lot
 * auth settings migration to config_plugins

lib/setup.php

@@ -147,21 +147,7 @@ global $THEME;
 
 
 /// Load up any configuration from the config table
-
-    if ($configs = get_records('config')) {
-        $CFG = (array)$CFG;
-        foreach ($configs as $config) {
-            if (!isset($CFG[$config->name])) {
-                $CFG[$config->name] = $config->value;
-            } else {
-                error_log("\$CFG->$config->name in config.php overrides database setting");
-            }
-        }
-
-        $CFG = (object)$CFG;
-        unset($configs);
-        unset($config);
-    }
+    $CFG = get_config();
 
 /// Turn on SQL logging if required
     if (!empty($CFG->logsql)) {