Don't use the libspf caching layer.

Caching should be left up to the (local) resolver, not to random libraries.
What's especially frustrating is that it also caches errors for abitrary
amounts of time

spf2.go

@@ -41,7 +41,7 @@ type clientImpl struct {
 // NewClient creates a new SPF client.
 func NewClient() Client {
 	client := new(clientImpl)
-	client.s = C.SPF_server_new(C.SPF_DNS_CACHE, 0)
+	client.s = C.SPF_server_new(C.SPF_DNS_RESOLV, 0)
 	return client
 }