tcp: Prepare sk-inet for dumping and restoring tcp connections

First of all -- to make crtools dump/restore established tcp sockets
you have to specify the --tcp-established options. By doing so you
inform crtools that

a) you know, that after dump there will be a netfilter rules blocking
   the dumped connections

b) you guarantee, that before restore this netfilter block is still
   there

What else this patch does is simple -- collects establised sockets and
calls the dump/restore tcp function (now empty) where appropriate.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>