files: Do setup_newborn_fds() later

This patch makes the call of service fds relocation after
root_prepare_shared()->prepare_fd_pid(). Next patches
will make service_fd_base depend on task's max fd used,
and for root_item we need to read all fles to know
the maximum of them.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>