base-manager: automatic required subsystem detection

Instead of assuming we require a fixed set of subsystems to monitor,
compile the full list based on what the plugins have requested
themselves.

src/mm-plugin.c

@@ -146,6 +146,12 @@ mm_plugin_get_name (MMPlugin *self)
     return self->priv->name;
 }
 
+const gchar **
+mm_plugin_get_allowed_subsystems (MMPlugin *self)
+{
+    return (const gchar **) self->priv->subsystems;
+}
+
 const gchar **
 mm_plugin_get_allowed_udev_tags (MMPlugin *self)
 {