make --enforce-mut-vars always on, add mut annotations to remaining files

src/rustdoc/rustdoc.rs

@@ -30,7 +30,7 @@ fn run_passes(
          original through each pass"
     )];
 
-    let passno = 0;
+    let mut passno = 0;
     vec::foldl(doc, passes) {|doc, pass|
         log(debug, #fmt("pass #%d", passno));
         passno += 1;