isDirectory() is not compatible with z/OS

[laijonathan]

    isDirectory() {
      return (this.mode & 61440) === 16384;
    }

On z/OS the isDirectory() function always returns false.
The inode masks S_IFMT and S_IFDIR (61440 and 16384) do not seem to be compatible since S_IFMT on z/OS is defined as 0xFF000000

External source: https://www.nntp.perl.org/group/perl.mvs/2014/11/msg1875.html

A potential fix I have tested is to use the constants from the fs module instead of a hardcoded value. eg: fsNode.constants.S_IFMT

[laijonathan]

@joliss @stefanpenner

Hi, I've made a PR that fixes #72

It would be awesome if we could get it reviewed. Thanks!