libct: server -- Make sure the socket is initialized

In case of start listening for commands make sure
the libct_session_export_prepare was called before
and socket is listening.

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

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