Support adding devices by dir

This enables cases where devices exist in a subdirectory of /dev,
particularly where those device names are not portable across machines,
which makes it problematic to specify from a runtime such as cri.

Added this to `ctr` as well so I could test that the code at least
works.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/ctr/commands/commands.go

@@ -153,7 +153,7 @@ var (
 		},
 		cli.StringSliceFlag{
 			Name:  "device",
-			Usage: "add a device to a container",
+			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",
 		},
 		cli.BoolFlag{
 			Name:  "seccomp",