Move extensions.LabelSelector to unversioned.

Move type LabelSelector and type LabelSelectorRequirement from pkg/apis/extensions
This avoids an import loop when Job (and later DaemonSet, Deployment, ReplicaSet)
are moved out of extensions to new api groups.

Also Move LabelSelectorAsSelector utility from pkg/apis/extensions/ to pkg/api/unversioned/
Also its test.
Also LabelSelectorOp* constants.
Also the pkg/apis/extensions/validation functions ValidateLabelSelectorRequirement and
ValidateLabelSelector move to pkg/api/unversioned

The related type in pkg/apis/extensions/v1beta1/ is staying there.  I might move
it in another PR if neccessary.

pkg/kubectl/run_test.go

@@ -22,6 +22,7 @@ import (
 
 	"k8s.io/kubernetes/pkg/api"
 	"k8s.io/kubernetes/pkg/api/resource"
+	"k8s.io/kubernetes/pkg/api/unversioned"
 	"k8s.io/kubernetes/pkg/apis/extensions"
 )
 
@@ -746,7 +747,7 @@ func TestGenerateJob(t *testing.T) {
 					Labels: map[string]string{"foo": "bar", "baz": "blah"},
 				},
 				Spec: extensions.JobSpec{
-					Selector: &extensions.LabelSelector{
+					Selector: &unversioned.LabelSelector{
 						MatchLabels: map[string]string{"foo": "bar", "baz": "blah"},
 					},
 					Template: api.PodTemplateSpec{