go.sys: add Unsetenv

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/153810043

windows/env_unset.go

@@ -0,0 +1,14 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.4
+
+package windows
+
+import "syscall"
+
+func Unsetenv(key string) error {
+	// This was added in Go 1.4.
+	return syscall.Unsetenv(key)
+}