capabilities: CAP_CHOWN might be required by many plugins opening UNIX sockets

But as the sockets will be created with the user/group of the running
process this might not be required as no change may be needed.

src/libcharon/plugins/error_notify/error_notify_plugin.c

@@ -92,6 +92,12 @@ plugin_t *error_notify_plugin_create()
 {
 	private_error_notify_plugin_t *this;
 
+	if (!lib->caps->keep(lib->caps, CAP_CHOWN))
+	{	/* required to chown(2) notify socket */
+		DBG1(DBG_CFG, "error-notify plugin requires CAP_CHOWN capability");
+		return NULL;
+	}
+
 	INIT(this,
 		.public = {
 			.plugin = {