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

Uses upstream go code, lightly adjusted.

Adds smoke test.

src/os/file_other.go

@@ -56,3 +56,7 @@ func putchar(c byte)
 func Pipe() (r *File, w *File, err error) {
 	return nil, nil, ErrNotImplemented
 }
+
+func tempDir() string {
+	return "/tmp"
+}