mount: create a mount point for the root mount namespace in the roots yard

These chnages allows us to:
* avoid difference between the root mount namespace and other mount namespaces
* support a read-only root mount
* don't create temporary directories in the root mount

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>