Move /nat to /pkg/nat

By convention /pkg is safe to use from outside the docker tree, for example
if you're building a docker orchestrator.

/nat currently doesn't have any dependencies outside of /pkg, so it seems
reasonable to move it there.

This rename was performed with:

```
gomvpkg -vcs_mv_cmd="git mv {{.Src}} {{.Dst}}" \
	-from github.com/docker/docker/nat \
        -to   github.com/docker/docker/pkg/nat

```

Signed-off-by: Peter Waller <p@pwaller.net>

integration-cli/docker_cli_create_test.go

@@ -9,7 +9,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/docker/docker/nat"
+	"github.com/docker/docker/pkg/nat"
 	"github.com/go-check/check"
 )