ipv6 bugfix, need review.

[Bruce Simpson]

Alfred Perlstein wrote:
>   The traffic class byte is set to 0x00000000 in the header of some
>   BGP packets sent between interfaces that have IPv6 addresses,
>   instead of the correct setting 0xc0 (INTERNETCONTROL).
>
>   

Content free argument:
    Feels right. I had to commit a man page diff to document the TCLASS 
socket option.
    If this isn't happening for TCP sockets, then that certainly needs 
dealing with.