syscall: handle empty environment variable values properly on windows

Setenv("AN_ENV_VAR", "") deletes AN_ENV_VAR instead of setting it
to "" at this moment. Also Getenv("AN_ENV_VAR") returns "not found",
if AN_ENV_VAR is "". Change it, so they behave like unix.

Fixes issue 5610

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/10594043
runtime: remove unused typedef

R=golang-dev, dave, r
CC=golang-dev
https://codereview.appspot.com/10660044
crypto/sha512: update comment for the Sum512 function.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/10660043

Committer: Dave Cheney <dave@cheney.net>