IPv6 transport class suppport

IPv6 transport class suppport

IPv6 supports the same concept of differentiated service for routing
protocols as IPv4, but like too many things, the standards committee
decided that having two names for the same thing wasn't good enough and
introduced a third more generic term transport class.

The socket option to set transport class works the same as IPv4, but the
arguments are different.

* lib/sockopt.[ch]
  * setsockopt_ipv6_tclass(): new function
* bgpd/bgp_network.c
  * bgp_connect(): set socket option
  * bgp_listener(): set socket option
* ospf6d/ospf6_network.c
  * ospf6_set_transport_class(): new function
  * ospf6_serv_sock(): set socket option
* ripngd/ripngd.c
  * ripng_make_socket(): set socket option