* use new ddcCorpusList::rfind() for CConcIndexator::LoadMaskedFiles()
- string-based file-masking only masks the LAST occurrence of filename key (rather than the FIRST)
- backwards-compatible for old-style unique-filename corpora
- should also Do The Right Thing for "replaced" files snuck in by dstar-ddc-update.perl
- more precise control of file-masking possible with binary CORPUS._masked_ids