Prevent unload of modules which implement an Optional API.

Once an Optional API module is loaded it should stay loaded.  Unloading
an optional API module runs the risk of a crash if something else is
using it.  This patch causes all optional API providers to tell the
module loader not to unload except at shutdown.

ASTERISK-27389

Change-Id: Ia07786fe655681aec49cc8d3d96e06483b11f5e6

res/res_http_websocket.c

@@ -1446,6 +1446,9 @@ static int load_module(void)
 	ast_http_uri_link(&websocketuri);
 	websocket_add_protocol_internal("echo", websocket_echo_callback);
 
+	/* For Optional API. */
+	ast_module_shutdown_ref(AST_MODULE_SELF);
+
 	return 0;
 }