fs: add access() and accessSync()

[cjihrig]

fs.exists() and fs.existsSync() do not follow typical node conventions. access() and accessSync() are added as alternatives in this commit.

@trevnorris I opened this as a separate PR because it was decided in #8418 that exists() and existsSync() would not be deprecated yet. One thought that I had is that these functions might seem less C-like if they returned true on success instead of the 0 returned by libuv. Thoughts?

[trevnorris]

I would just say "fs.exists() will be deprecated." since we don't recommend using anything to check for a file exists then blindly reading from it (which is how fs.exists() is being used).

[trevnorris]

Awesome stuff. Thanks for getting this in all the way from libuv to here.

Just a few comments, but LGTM.

[trevnorris]

@cjihrig

One thought that I had is that these functions might seem less C-like if they returned true on success instead of the 0 returned by libuv.

Good point. There are a couple different ways this could work. Honestly there wouldn't have to be a return value at all, since if everything is fine then no error object will exist. e.g.

fs.access('/some/thing', function(er) {
  if (!er) {
    // The access check must have succeeded.
  }
});

And for fs.accessSync() the user doesn't even need to check the return value since it'll throw otherwise.

Though this type of API change might be drastic. I definitely like that it'll return a useful error (e.g. ENOENT or ENOTDIR). For now just change it to true/false.

[cjihrig]

@trevnorris updated based on your comments.

[timoxley]

bump.

[trevnorris]

Thanks for the bump. Landed in 2944934.