Fix unique files not appearing if HEU_LVL>0.

Fix unique files that do not start a sequence not appearing in the list
 (unique_lines was appended via newline to itself instead of
  to LIST_TO_CHOOSE_FROM). Also fix numbering for those lines and high-
  liting KEYWORD in the filenames. Start using GRE for grep.

Fix check for setting HEURISTICS_LEVEL a value higher than
  MAX_HEURISTICS_LEVEL.