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: nodejs#17921

PR-URL: nodejs#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

@@ -234,14 +234,10 @@ fs.exists = function(path, callback) {
   }
 
   try {
-    path = getPathFromURL(path);
-    validatePath(path);
+    fs.access(path, fs.FS_OK, suppressedCallback);
   } catch (err) {
     return callback(false);
   }
-  var req = new FSReqWrap();
-  req.oncomplete = suppressedCallback;
-  binding.stat(pathModule.toNamespacedPath(path), req);
 };
 
 Object.defineProperty(fs.exists, internalUtil.promisify.custom, {
@@ -260,13 +256,7 @@ Object.defineProperty(fs.exists, internalUtil.promisify.custom, {
 // TODO(joyeecheung): deprecate the never-throw-on-invalid-arguments behavior
 fs.existsSync = function(path) {
   try {
-    path = getPathFromURL(path);
-    validatePath(path);
-    const ctx = { path };
-    binding.stat(pathModule.toNamespacedPath(path), undefined, ctx);
-    if (ctx.errno !== undefined) {
-      return false;
-    }
+    fs.accessSync(path, fs.FS_OK);
     return true;
   } catch (e) {
     return false;