cr-restore: restore root sid before mount namespace

This is a precursor patch for AutoFS mount restore.
To restore AutoFS indirect mount points, we have to create mount point dentries.
This can do only the process with pgrp, configured to mount. Process, which
does AutoFS mount call (init) at that moment is not.
Restoring actual pgrp before sid breaks sid restoring.
Thus, restoring of sid is required to make process group leader.

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>