WL#2573: remove Natural Language Full-Text Search limitations from boolean mode

The result of the full-text search in boolean mode should contain ALL rows that
satisfy the expression.

In particular that means that

  * stopword list should have no effect
  * ft_{mix,max}_word_len should have no effect
  * search for "-word" should return all rows that do not contain the word

This means that if MySQL cannot use full-text index to resolve the query it
should resort to full table scan (just as it works for regular indexes).

Perhaps there should be a server variable (global and local) to disable this
mode - as full-text queries are usually typed in by users, making them
fall back to a table scan can easily DoS a server.