WL#2559: Enable fulltext search for non-MyISAM engines

RATIONALE

Needed to make a framework for all engines (that implement native
full-text indexes) to support full-text search.

SUMMARY

Most of the full-text search functionality should be moved from MyISAM one level
up and be storage engine independent.

This task will also make it possible to use boolean search on tables that don't
support full-text indexes, and on columns from different tables.

Fulltext search code will be split in two parts: parsing and searching

=============================

The parser will be pure C, it'll be in the mysys/ft_parser.c
To parse a row for indexing storage engine will need to do:

  ft_parse_init();
  for_every_keyseg { ft_parse(); }
  ft_parse_end();

basically it's what _mi_ft_parserecord does now.
ft_parse_end() is a new name of ft_linearize(). but besides
renaming API will be almost the same it is now.

=============================

Search will be C++, via handler interface.

  handler::ft_count(word)
    returns a number of rows this word is present in (MyISAM
    returns a wild guess if it does not know)

  handler::ft_search_word(word)
  handler::ft_search_next()
    searches by a word or a prefix, returns position (old
    name - docid) and weight, but NOT a record if
    word==NULL, last used word is used (an optimization when
    ft_search_word is called right after ft_count)

  handler::rnd_pos()
     used to read a record by position, as usual

  methods of records.cc could be reused, especially
  rr_from_cache is interesting
                         
=============================

ft2 as myisam specific optimization, stays in MyISAM
special code for concurrent inserts stays in MySAM

we assume that ft_search_word()/next returns words ordered
by position, then "smarter index-merge" optimization is
universally applicable

stopwords are removed from ft_static.c and put in a
plain-text file somewhere in share/

=============================

Directory structure:

 mysys/ft_parser.c
 mysys/ft_stopwords.c
 share/english/stopwords.txt

 scripts/ftbench/ - moved verbatim
 (other suggestions where to put ftbench are welcome)

 sql/item_ftfunc.* - relevant Item's from Item_func.*
                     and ft_nlq_search.c/ft_boolean_search.c

 include/fulltext.h - myisam-independent part of old ftdefs.h

 myisam/ft_eval.* - it's deleted, as obsolete
 myisam/ft_test1.* - it's deleted, as obsolete
 myisam/ft_stem.c - deleted

 myisam/ftdefs.h    - myisam-specific part of old ftdefs.h
 myisam/fulltext.h  - merged with what remained from myisam/ftdefs.h
 myisam/ft_update.c - not moved. myisam-specific code
 myisam/ft_static.c - not moved. myisam-specific code (no stopwords there)