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

`tmpdir()` was introduced as replacement 3 years ago in
3fe6aba

PR-URL: #6739
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Fishrock123

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();