luci-base: dispatcher: add login indication on 403 errors

Send a custom LuCI X-Header to indicate that a login is required to access
the requested resource. This is mainly intended for xhr.js to be able to
intercept such responses and popup an authentication dialog.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/luasrc/dispatcher.lua

@@ -417,6 +417,7 @@ function dispatch(request)
 				context.path = {}
 
 				http.status(403, "Forbidden")
+				http.header("X-LuCI-Login-Required", "yes")
 				tmpl.render(track.sysauth_template or "sysauth", {
 					duser = default_user,
 					fuser = user
@@ -433,6 +434,7 @@ function dispatch(request)
 
 		if not sid or not sdat then
 			http.status(403, "Forbidden")
+			http.header("X-LuCI-Login-Required", "yes")
 			return
 		end