netfilter: xtables: give xt_ecn its own name

Use the new macro and struct names in xt_ecn.h, and put the old
definitions into a definition-forwarding ipt_ecn.h.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>