WL#639: rework stopword handling in fulltext search

Originally stopwords were stored in the index. I don't remember why I changed
this - obviously, there were some problems with this approach. Still it offers
so many benefits, that I'll try to implement it again.

Another approach could be to store _path to stopword list_ in MYI/frm file
well, frm file has to store only the path anyway.
yep, and MYI file needs to store the path too - for REPAIR/myisamchk to work :(