Implement --device idType://id for ctr run on Windows

Also fixes the issue that `ctr run` on Windows offered help for the
non-Windows implementation, but was silently ignored.

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

cmd/ctr/commands/commands_unix.go

@@ -40,5 +40,8 @@ func init() {
 	}, cli.StringFlag{
 		Name:  "rootfs-propagation",
 		Usage: "set the propagation of the container rootfs",
+	}, cli.StringSliceFlag{
+		Name:  "device",
+		Usage: "file path to a device to add to the container; or a path to a directory tree of devices to add to the container",
 	})
 }