[DCCP]: Introduce DCCPv6

Still needs mucho polishing, specially in the checksum code, but works
just fine, inet_diag/iproute2 and all 8)

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>