New init function for plugins, supports errors reporting

All plugins can optionally add an init function named `pluginname_init()` which is called when the plugin is loaded.

This function is aware of the config, and can return initialization errors, which are displayed in the header template.

Note that the previous error system hack no longer work.

plugins/demo_plugin/demo_plugin.php

@@ -14,6 +14,23 @@
  * and check user status with _LOGGEDIN_.
  */
 
+/**
+ * Initialization function.
+ * It will be called when the plugin is loaded.
+ * This function can be used to return a list of initialization errors.
+ *
+ * @param $conf ConfigManager instance.
+ *
+ * @return array List of errors (optional).
+ */
+function demo_plugin_init($conf)
+{
+    $conf->get('toto', 'nope');
+
+    $errors[] = 'This a demo init error.';
+    return $errors;
+}
+
 /**
  * Hook render_header.
  * Executed on every page redering.