Fixes#1115.
Also improves the performances of DiskArchive’s directory
implementation a lot, simply by not going through the entire tree
instead of just listing the first level files.
Thanks to JayRoxFox for rebasing this on current master!
ScanDirectoryTreeAndCallback, before this change, coupled error/return
codes and actual return values (number of entries found). This caused
confusion and difficulty interpreting the precise way the function
worked.
Supersedes, and closes#1255.
