linux: Honor RuncOptions if set on container

This also fix the type used for RuncOptions.SystemCgroup, hence introducing
an API break.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

linux/shim/client_unix.go

@@ -3,7 +3,6 @@
 package shim
 
 import (
-	"context"
 	"os/exec"
 	"syscall"
 )
@@ -12,6 +11,6 @@ var atter = syscall.SysProcAttr{
 	Setpgid: true,
 }
 
-func setCgroup(ctx context.Context, config Config, cmd *exec.Cmd) error {
+func setCgroup(cgroupPath string, cmd *exec.Cmd) error {
 	return nil
 }