util-net: Itroduce send_fds/recv_fds routines

We will need these helpers to transfer file
descriptors from dumpee to our space.

Also make send_fd/recv_fd to be a wrappers over
send_fds/revc_fds to not duplicate the code.

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