Some changes to support the new --filter option:

- Changed XFLG_WORDS_ONLY to XFLG_NO_PREFIXES.
- Added XFLG_DEF_EXCLUDE and XFLG_ABS_PATH.
- Added some new MATCHFLG_* values for the new merge-file support.
- Put the "slash_cnt" var (in the exclude_struct) into a union with
  the new megelist pointer.