runc: implement --console-socket

This allows for higher-level orchestrators to be able to have access to
the master pty file descriptor without keeping the runC process running.
This is key to having (detach && createTTY) with a _real_ pty created
inside the container, which is then sent to a higher level orchestrator
over an AF_UNIX socket.

This patch is part of the console rewrite patchset.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

exec.go

@@ -29,6 +29,10 @@ following will output a list of processes running in the container:
 
        # runc exec <container-id> ps`,
 	Flags: []cli.Flag{
+		cli.StringFlag{
+			Name:  "console-socket",
+			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the master end of the console's pseudoterminal",
+		},
 		cli.StringFlag{
 			Name:  "cwd",
 			Usage: "current working directory in the container",
@@ -127,6 +131,7 @@ func execProcess(context *cli.Context) (int, error) {
 		enableSubreaper: false,
 		shouldDestroy:   false,
 		container:       container,
+		consoleSocket:   context.String("console-socket"),
 		detach:          detach,
 		pidFile:         context.String("pid-file"),
 	}