fs: use fs.access in fs.exists

Uses fs.access to implement fs.exists functionality. Fixes a issue,
when a file exists but user does not have privileges to do stat on the
file.

Fixes: #17921

PR-URL: #18618
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/fs.js

@@ -323,12 +323,7 @@ fs.accessSync = function(path, mode) {
 };
 
 fs.exists = function(path, callback) {
-  if (handleError((path = getPathFromURL(path)), cb))
-    return;
-  if (!nullCheck(path, cb)) return;
-  var req = new FSReqWrap();
-  req.oncomplete = cb;
-  binding.stat(pathModule.toNamespacedPath(path), req);
+  fs.access(path, fs.F_OK, cb);
   function cb(err) {
     if (callback) callback(err ? false : true);
   }
@@ -345,9 +340,7 @@ Object.defineProperty(fs.exists, internalUtil.promisify.custom, {
 
 fs.existsSync = function(path) {
   try {
-    handleError((path = getPathFromURL(path)));
-    nullCheck(path);
-    binding.stat(pathModule.toNamespacedPath(path));
+    fs.accessSync(path, fs.F_OK);
     return true;
   } catch (e) {
     return false;