ctr: Add Linux Capabilities control flags

This allows Linux Capabilities to be modified via `ctr`.

e.g.
```
$ sudo ./bin/ctr run --cap-add "CAP_SYS_ADMIN" --cap-drop "CAP_NET_RAW" \
--rm docker.io/library/busybox:latest foo cat /proc/self/status | grep Cap
CapInh: 00000000a82405fb
CapPrm: 00000000a82405fb
CapEff: 00000000a82405fb
CapBnd: 00000000a82405fb
CapAmb: 0000000000000000
```

Signed-off-by: Manabu Sugimoto <Manabu.Sugimoto@sony.com>

cmd/ctr/commands/commands.go

@@ -169,6 +169,14 @@ var (
 			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",
 		},
+		cli.StringSliceFlag{
+			Name:  "cap-add",
+			Usage: "add Linux capabilities (Set capabilities with 'CAP_' prefix)",
+		},
+		cli.StringSliceFlag{
+			Name:  "cap-drop",
+			Usage: "drop Linux capabilities (Set capabilities with 'CAP_' prefix)",
+		},
 		cli.BoolFlag{
 			Name:  "seccomp",
 			Usage: "enable the default seccomp profile",