encoding/json: document case-insensitive Unmarshal key matching

Fixes issue 4664.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7237060
cmd/ld: retry short writes, to get error detail

Fixes issue 3802.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7228066