Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729

Reduce some unnecessary overhead.

Replaces StatFile(), with a portable data object.

PathExists() should better do what CheckAccess() does, and
CheckAccess() doesn't do what its name implies.

std::auto_ptr is deprecated, and std::unique_ptr is much better
anyway.

The old struct config_param remains only for top-level string options.