go.net/ipv4: make IANA registry parser robust
go.net/ipv4: make IANA registry parser robust

- specify complete resource path to avoid receiving xhtml
- make use of keyword instead of table index

R=dave
CC=golang-dev
https://codereview.appspot.com/10700045

polydawn-ports/go.net,gigq/go.net,ssor/go.net,go-mirrors/go.net,fernandez14/go.net