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>

nat/sort_test.go

@@ -0,0 +1,85 @@
+package nat
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+func TestSortUniquePorts(t *testing.T) {
+	ports := []Port{
+		Port("6379/tcp"),
+		Port("22/tcp"),
+	}
+
+	Sort(ports, func(ip, jp Port) bool {
+		return ip.Int() < jp.Int() || (ip.Int() == jp.Int() && ip.Proto() == "tcp")
+	})
+
+	first := ports[0]
+	if fmt.Sprint(first) != "22/tcp" {
+		t.Log(fmt.Sprint(first))
+		t.Fail()
+	}
+}
+
+func TestSortSamePortWithDifferentProto(t *testing.T) {
+	ports := []Port{
+		Port("8888/tcp"),
+		Port("8888/udp"),
+		Port("6379/tcp"),
+		Port("6379/udp"),
+	}
+
+	Sort(ports, func(ip, jp Port) bool {
+		return ip.Int() < jp.Int() || (ip.Int() == jp.Int() && ip.Proto() == "tcp")
+	})
+
+	first := ports[0]
+	if fmt.Sprint(first) != "6379/tcp" {
+		t.Fail()
+	}
+}
+
+func TestSortPortMap(t *testing.T) {
+	ports := []Port{
+		Port("22/tcp"),
+		Port("22/udp"),
+		Port("8000/tcp"),
+		Port("6379/tcp"),
+		Port("9999/tcp"),
+	}
+
+	portMap := PortMap{
+		Port("22/tcp"): []PortBinding{
+			{},
+		},
+		Port("8000/tcp"): []PortBinding{
+			{},
+		},
+		Port("6379/tcp"): []PortBinding{
+			{},
+			{HostIp: "0.0.0.0", HostPort: "32749"},
+		},
+		Port("9999/tcp"): []PortBinding{
+			{HostIp: "0.0.0.0", HostPort: "40000"},
+		},
+	}
+
+	SortPortMap(ports, portMap)
+	if !reflect.DeepEqual(ports, []Port{
+		Port("9999/tcp"),
+		Port("6379/tcp"),
+		Port("8000/tcp"),
+		Port("22/tcp"),
+		Port("22/udp"),
+	}) {
+		t.Errorf("failed to prioritize port with explicit mappings, got %v", ports)
+	}
+	if pm := portMap[Port("6379/tcp")]; !reflect.DeepEqual(pm, []PortBinding{
+		{HostIp: "0.0.0.0", HostPort: "32749"},
+		{},
+	}) {
+		t.Errorf("failed to prioritize bindings with explicit mappings, got %v", pm)
+	}
+}