(plugins) work in progress interface infrastructure

Plugins should be able to register new interfaces.
The basic idea is to materialize interface settings into the running config, so other services can utilize them in a uniform way.
Current hooks for legacy vpns, ipsec and openvpn should move into the same structure, which eliminates current static hooks.

The return content of the interface type itself needs to be further refined.

src/etc/inc/plugins.inc

@@ -44,6 +44,56 @@ function plugins_services()
     return $services;
 }
 
+/**
+ * Register new or changed interfaces into config's interfaces section.
+ * Every <plugin>_interface should return a named array containing the interface unique identifier and properties.
+ *
+ */
+function plugins_register_interfaces()
+{
+    global $config;
+    $changed_interfaces = array();
+
+    // register / update interfaces
+    foreach (plugin_scan() as $name => $path) {
+        require_once $path;
+        $func = sprintf('%s_interface', $name);
+        if (function_exists($func)) {
+            foreach ($func() as $intf_ref => $intf_data) {
+                if (is_array($intf_data)) {
+                    if (empty($config['interfaces'][$intf_ref])) {
+                        $config['interfaces'][$intf_ref] = array();
+                    }
+                    $config['interfaces'][$intf_ref]['internal_dynamic'] = true;
+                    // traverse and diff interface properties with known configuration
+                    $intf_config = &$config['interfaces'][$intf_ref];
+                    foreach ($intf_data as $prop_name => $prop_value) {
+                        if (empty($intf_config[$prop_name]) || $intf_config[$prop_name] != $prop_value) {
+                            $intf_config[$prop_name] = $prop_value;
+                            if (!in_array($intf_ref, $changed_interfaces)) {
+                                $changed_interfaces[] = $intf_ref;
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    // cleanup registrations
+    foreach ($config['interfaces'] as $intf => $intf_data) {
+        if (!empty($intf_data['internal_dynamic']) && !in_array($intf, $changed_interfaces)) {
+            $changed_interfaces[] = $intf;
+            unset($config['interfaces'][$intf]);
+        }
+    }
+
+    // configuration changed, materialize
+    if (count($changed_interfaces) > 0) {
+        write_config();
+    }
+}
+
 function plugins_configure()
 {
     foreach (plugin_scan() as $name => $path) {