runc exec: fix setting process.Scheduler

Commit 770728e1 added Scheduler field into both Config and Process,
but forgot to add a mechanism to actually use Process.Scheduler.
As a result, runc exec does not set Process.Scheduler ever.

Fix it, and a test case (which fails before the fix).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/standard_init_linux.go

@@ -155,7 +155,7 @@ func (l *linuxStandardInit) Init() error {
 		}
 	}
 
-	if err := setupScheduler(l.config.Config); err != nil {
+	if err := setupScheduler(l.config); err != nil {
 		return err
 	}