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>

registry/auth/auth.go

@@ -61,12 +61,12 @@ type Access struct {
 // header values based on the error.
 type Challenge interface {
 	error
-	// ServeHTTP prepares the request to conduct the appropriate challenge
-	// response by adding the appropriate HTTP challenge header on the response
-	// message. Callers are expected to set the appropriate HTTP status code
-	// (e.g. 401) themselves. Because no body is written, users may write a
-	// custom body after calling ServeHTTP.
-	ServeHTTP(w http.ResponseWriter, r *http.Request)
+
+	// SetHeaders prepares the request to conduct a challenge response by
+	// adding the an HTTP challenge header on the response message. Callers
+	// are expected to set the appropriate HTTP status code (e.g. 401)
+	// themselves.
+	SetHeaders(w http.ResponseWriter)
 }
 
 // AccessController controls access to registry resources based on a request