ftbw->off wasn't cleared on reinit - Bug#8234


include/ft_global.h:
  get rid of default_charset_info in indexless fulltext searches
myisam/ft_boolean_search.c:
  get rid of default_charset_info in indexless fulltext searches
  clear ftbw->off on reinits
myisam/ft_static.c:
  get rid of default_charset_info in indexless fulltext searches
myisam/ftdefs.h:
  get rid of default_charset_info in indexless fulltext searches
sql/ha_myisam.h:
  get rid of default_charset_info in indexless fulltext searches
sql/handler.h:
  get rid of default_charset_info in indexless fulltext searches
sql/item_func.cc:
  get rid of default_charset_info in indexless fulltext searches