cmd/{ctr,dist}: label can be a verb

Rather than using the more verbose `set-labels` command, we are changing
the command to set labels for various objects to `label`, as it can be
used as a verb. This matches changes in the content store labeling.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/ctr/namespaces.go

@@ -54,7 +54,7 @@ var namespacesCreateCommand = cli.Command{
 }
 
 var namespacesSetLabelsCommand = cli.Command{
-	Name:        "set-labels",
+	Name:        "label",
 	Usage:       "Set and clear labels for a namespace.",
 	ArgsUsage:   "[flags] <name> [<key>=<value>, ...]",
 	Description: "Set and clear labels for a namespace.",