util: add ability to change a file descriptor on procfs (v2)

When a pidns is dumped, crtools uses procfs from this pidns
for getting information about zombies.

v2: Use close_proc() instead of restore_proc_fd().
    A current proc will be opened again, if it's required.

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