make docker-credential-wincred work like docker-credential-osxkeychain

* fetch credentials for server with matching hostname if scheme, path, or port are not provided
* if the credential request includes specific scheme, path, or port that does not match entry, don't return
* extract url helpers into a package

Signed-off-by: Emily Casey <ecasey@pivotal.io>
Signed-off-by: Danny Joyce <djoyce@pivotal.io>

registryurl/url_go18.go

@@ -0,0 +1,15 @@
+//+build go1.8
+
+package registryurl
+
+import (
+	url "net/url"
+)
+
+func GetHostname(u *url.URL) string {
+	return u.Hostname()
+}
+
+func GetPort(u *url.URL) string {
+	return u.Port()
+}