sockets, unix: Use general machnism for checkpoint/restore v2

Use fdtype_ops facility to c/r unix sockets.

v2
 - BUG_ON added in dump_one_unix_fd if socket
   is already dumped since we never should dump
   same socket several times
 - The order of restore remains as it was before,
   the lookup is done via socket inode numbers

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