gui, lib/api: Use effective listen address for no auth warning

This adds a field `guiAddressUsed` to the system status response, that
holds the current listening address actually in use. This may be
different from the one stored in the config because it may have been
overridden by environment or command line flag.

The GUI now checks this field to see if we are listening on localhost.
If we are not, the authentication required warning is displayed,
regardless of the *configured* listening address.

lib/api/api.go

@@ -913,6 +913,7 @@ func (s *service) getSystemStatus(w http.ResponseWriter, r *http.Request) {
 	res["uptime"] = s.urService.UptimeS()
 	res["startTime"] = ur.StartTime
 	res["guiAddressOverridden"] = s.cfg.GUI().IsOverridden()
+	res["guiAddressUsed"] = s.cfg.GUI().Address()
 
 	sendJSON(w, res)
 }