soccr/tcp: Add _pause and _resume to library and use it

The calls put socket into a mode where it can be saved or
restored. Add take one out of this mode.

TODO: In criu we turn repair off (resume the socket) from
restorer blob that cannot use the library. Need to somehow
fix it.

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