Dump and restore current work directory (v2)

CWD is saved as file descriptor with number -1.

v2: use dump_regular_file

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>