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>

libcontainer/console_linux.go

@@ -36,8 +36,8 @@ type linuxConsole struct {
 	slavePath string
 }
 
-func (c *linuxConsole) Fd() uintptr {
-	return c.master.Fd()
+func (c *linuxConsole) File() *os.File {
+	return c.master
 }
 
 func (c *linuxConsole) Path() string {