Simplify auth.Challenge interface to SetHeaders

This removes the erroneous http.Handler interface in favor a simple SetHeaders
method that only operattes on the response. Several unnecessary uses of pointer
types were also fixed up.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/auth/silly/access_test.go

@@ -21,7 +21,7 @@ func TestSillyAccessController(t *testing.T) {
 		if err != nil {
 			switch err := err.(type) {
 			case auth.Challenge:
-				err.ServeHTTP(w, r)
+				err.SetHeaders(w)
 				w.WriteHeader(http.StatusUnauthorized)
 				return
 			default: