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://golang.org/cl/10700045

albertjin/golang-net,subuk/net,mckinney-subgraph/net,chenzlabs/net,dafyddcrosby/net,raboof/net,cmars/tools,cmars/tools,cmars/tools,veliamsli/net,foreversmart/net,veliamsli/net,kiip/net,zabawaba99/net,subuk/net,muzining/net,raboof/net,kiip/net,zabawaba99/net,yangxianjun/net,w4ngyi/net,DramaFever/golang.org-x-net,mckinney-subgraph/net,foreversmart/net,yangxianjun/net,muzining/net,w4ngyi/net,DramaFever/golang.org-x-net,flowlo/go-net,chenzlabs/net,gomini/net,albertjin/golang-net,dafyddcrosby/net,flowlo/go-net,golang/net,gomini/net,cmars/tools,cesanta/goxnet,cesanta/goxnet