os, syscall: add Unsetenv

Also address a TODO, making Clearenv pass through to cgo.

Based largely on Minux's earlier https://codereview.appspot.com/82040044

Fixes Issue 6423

LGTM=iant, alex.brainman, r, rsc
R=rsc, iant, r, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/148370043