Windows: Default to npipe transport

Signed-off-by: John Starks <jostarks@microsoft.com>

client/client_darwin.go

@@ -1,5 +1,3 @@
-// +build windows darwin
-
 package client
 
 // DefaultDockerHost defines os specific default if DOCKER_HOST is unset

client/client_windows.go

@@ -0,0 +1,4 @@
+package client
+
+// DefaultDockerHost defines os specific default if DOCKER_HOST is unset
+const DefaultDockerHost = "npipe:////./pipe/docker_engine"