IE compatibility mode tweaks

Set "X-UA-Compatible" meta tag (MSIE browsers only - forces browser compatibility mode)
JS based warning if Document Mode doesn't match IE version (MSIE browsers only)

core/src/main/java/hudson/Functions.java

@@ -202,7 +202,8 @@ public class Functions {
     }
     
     public static void initPageVariables(JellyContext context) {
-        String rootURL = Stapler.getCurrentRequest().getContextPath();
+        StaplerRequest currentRequest = Stapler.getCurrentRequest();
+        String rootURL = currentRequest.getContextPath();
 
         Functions h = new Functions();
         context.setVariable("h", h);
@@ -222,6 +223,8 @@ public class Functions {
          */
         context.setVariable("resURL",rootURL+getResourcePath());
         context.setVariable("imagesURL",rootURL+getResourcePath()+"/images");
+
+        context.setVariable("userAgent", currentRequest.getHeader("User-Agent"));
     }
 
     /**