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

Uses upstream go code, lightly adjusted.

Adds smoke test.

src/os/file_windows.go

@@ -1,8 +1,16 @@
+//go:build windows
 // +build windows
 
+// 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 "syscall"
+import (
+	"syscall"
+	"unicode/utf16"
+)
 
 type syscallFd = syscall.Handle
 
@@ -23,6 +31,24 @@ func Pipe() (r *File, w *File, err error) {
 	return
 }
 
+func tempDir() string {
+	n := uint32(syscall.MAX_PATH)
+	for {
+		b := make([]uint16, n)
+		n, _ = syscall.GetTempPath(uint32(len(b)), &b[0])
+		if n > uint32(len(b)) {
+			continue
+		}
+		if n == 3 && b[1] == ':' && b[2] == '\\' {
+			// Do nothing for path, like C:\.
+		} else if n > 0 && b[n-1] == '\\' {
+			// Otherwise remove terminating \.
+			n--
+		}
+		return string(utf16.Decode(b[:n]))
+	}
+}
+
 // 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.