[JENKINS-11172] Return HTTP 404 on /signup URL if not possible

core/src/main/resources/hudson/security/SecurityRealm/signup.jelly

@@ -23,6 +23,7 @@ THE SOFTWARE.
 -->
 <?jelly escape-by-default='true'?>
 <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form" xmlns:i="jelly:fmt">
+    <st:statusCode value="404" />
     <st:include page="sidepanel.jelly" from="${app}" it="${app}" />
     <l:layout title="${%Signup not supported}">
         <l:main-panel>