[DCCP]: Make CCID2 be the default

As per the draft. This fixes the build when netfilter dccp components
are built and dccp isn't. Thanks to Reuben Farrelly for reporting
this.

The following changesets will introduce /proc/sys/net/dccp/defaults/
to give more flexibility to DCCP developers and testers while apps
doesn't use setsockopt to specify the desired CCID, etc.

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