Disable the subreaper on exec

This keeps the flag but makes it hidden so that existing clients do not
encounter an error if we were to have removed the flag.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

exec.go

@@ -79,8 +79,9 @@ following will output a list of processes running in the container:
 			Usage: "add a capability to the bounding set for the process",
 		},
 		cli.BoolFlag{
-			Name:  "no-subreaper",
-			Usage: "disable the use of the subreaper used to reap reparented processes",
+			Name:   "no-subreaper",
+			Usage:  "disable the use of the subreaper used to reap reparented processes",
+			Hidden: true,
 		},
 	},
 	Action: func(context *cli.Context) error {
@@ -115,7 +116,7 @@ func execProcess(context *cli.Context) (int, error) {
 		return -1, err
 	}
 	r := &runner{
-		enableSubreaper: !context.Bool("no-subreaper"),
+		enableSubreaper: false,
 		shouldDestroy:   false,
 		container:       container,
 		console:         context.String("console"),