os: TempDir(): obey TMPDIR on unix, TMP on win, etc

Uses upstream go code, lightly adjusted.

Adds smoke test.

src/os/file_unix.go

@@ -1,5 +1,9 @@
 // +build darwin linux,!baremetal
 
+// Portions copyright 2009 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.
+
 package os
 
 import (
@@ -26,6 +30,14 @@ func Pipe() (r *File, w *File, err error) {
 	return
 }
 
+func tempDir() string {
+	dir := Getenv("TMPDIR")
+	if dir == "" {
+		dir = "/tmp"
+	}
+	return dir
+}
+
 // ReadAt reads up to len(b) bytes from the File starting at the given absolute offset.
 // It returns the number of bytes read and any error encountered, possibly io.EOF.
 // At end of file, Pread returns 0, io.EOF.