[physfs] enumerateFiles Regression on Mac OS

[Cheeseness]

Hi hi

I've found some unexpected behaviour with PHYSFS_enumerateFiles() on Mac
OS 10.10.5 with the current 2.1.0 development branch (as at changeset
1384:d844add9a935).

PHYSFS_enumerateFiles() will not list anything mounted outside of /,
including mounted subdirectories of /.

To reproduce,

  $ mkdir foo; touch foo/bar
  $ ./test_physfs
  > mount foo test 1
  > enumerate test

This will consistently return "total (0) files" with 2.1.0, but with the
2.0.3 stable release, "bar" is listed as being within the virtual folder
"test".

I haven't had time to look through and see if I can work out what's
going on, but I figured I'd make note of it here in case anybody with
more familiarity than me has time/enthusiasm enough to take a look
before I get around to it (it's low on my priority list since I the
stable build is functional enough for me right now).

Cheese