add support to kill container process by pid

This adds support for signalling a container process by pid.

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

make Ps more extensible

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

ps: windows support

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

cmd/ctr/main.go

@@ -63,6 +63,7 @@ containerd client
 		resumeCommand,
 		snapshotCommand,
 		versionCommand,
+		psCommand,
 	}
 	app.Commands = append(app.Commands, extraCmds...)
 	app.Before = func(context *cli.Context) error {