interface change for traversal to support union filesystems that use non-filename based ignore patterns

Changed interface between traversal & mediator/union to pass a callback
function instead of a set of filenames to ignore. The function implemented
for Aufs replicates the previous ignore_files functionality, but the new
interface can examine more than just the filename (useful for union/overlay
filesystems that denote special files by means other than filename, such as 
by using xattrs as in OverlayFS).