sk-inet: save the socket option IPV6_V6ONLY (v2)

Most part of services (ssh, httpd, ...) create two separate sockets
one for ipv4 and one for ipv6. If IPV6_V6ONLY isn't dumped, bind() returns
EADDRINUSE

v2: use do_dump_opt and initialize yes = 1

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>