Split out into a small stub command line and a library

- this is pretty much the smallest change to split this out and it
  exposes a few things that can be improved later
- no change to logging yet

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

src/moby/util_windows.go

@@ -0,0 +1,9 @@
+package moby
+
+import (
+	"os"
+)
+
+func homeDir() string {
+	return os.Getenv("USERPROFILE")
+}