IGNORE/USE/FORCE INDEX hints were honored when choosing FULLTEXT
index.

With this fix these hints are ignored. For regular indexes we may
perform table scan instead of index lookup when IGNORE INDEX was
specified. We cannot do this for FULLTEXT in NLQ mode.


mysql-test/r/fulltext.result:
  A test case for bug#25951.
mysql-test/t/fulltext.test:
  A test case for bug#25951.
sql/item_func.cc:
  IGNOR/USE/FORCE INDEX hints should not be honored when choosing FULLTEXT
  index.
  
  Use proper bitmap, that is not modified by IGNORE/USE/FORCE INDEX hints.