os: deprecate `tmpDir()` in favour of `tmpdir()`

`tmpdir()` was introduced as replacement 3 years ago in
3fe6aba558

PR-URL: https://github.com/nodejs/node/pull/6739
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/os.js

@@ -51,7 +51,8 @@ exports.tmpdir = function() {
   return path;
 };
 
-exports.tmpDir = exports.tmpdir;
+exports.tmpDir = internalUtil.deprecate(exports.tmpdir,
+    'os.tmpDir() is deprecated. Use os.tmpdir() instead.');
 
 exports.getNetworkInterfaces = internalUtil.deprecate(function() {
   return exports.networkInterfaces();