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 #5610

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10594043