DWDS/Dialing Concordance Code

* added -O/--options option for ddc_dump (dump *.opt)
* added DefaultQueryIndex clauses to CConcIndexator::SaveOptionsToString()

v2.1.25:
* fixed bug causing empty count results from count-queries with vacuous date- and/or bibl-sort filters
- example: count(* #asc_date) #by[textClass]
- problem was signed/unsigned comparison of index data (DWORD) with filter bounds (int m_KeyLo.i, m_KeyHi.i)
in Bibliography.cpp CConcXml::FileMatches(), CConcXml::CFreeBiblIndex::FileMatches()

+ ddc_opt.pod: made '1 opt-file per physical subcorpus' more explicit

v2.1.25-pre1: added support for invisible bibl fields in ddc_dump

* documentation tweaks

v2.1.24

v2.1.24-rc1
* 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

+ fixes for clean compilation under gcc v7.3.0 (e.g. ubuntu 18.04.1 LTS)

* new typedef CFileNo = DWORD
* new typedef CMaskedFileSet = set<CFileNo> for CConcIndexator::m_MaskedFiles
* propagate CConcIndexator::m_MaskedFiles correctly through ddc_union, ddc_split using new SaveMaskedFileIds() method

* propagate CORPUS._masked_ids through ddc_union