mount command should use its own path to find sub-commands

[rpaquay]

See https://github.com/chromium/axiom/blob/master/lib/wash/exe/mount.js#L88

  var fsMountCmd = new Path('jsfs:/exe/mount.' + fsType);

The code currently a hard-coded path. It should use cx.path as the basis for creating the name of the sub-mount commands.

[ussuri]

Makes sense. I'll fix this.