Fixed a bug in cvmfs_listxattr that was causing it to return a null
string in the middle of the list of extended attributes for regular
files.
Merge branch 'hotfix/2.1.11-jrandall-listxattr'
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 in other ways such as by using xattrs)
Merge branch 'feature/overlayfs' into feature/ignore-file-fn

Conflicts:
	cvmfs/sync_mediator.cc
	cvmfs/sync_mediator.h