WL#284: Fulltext: per-ft_index options

One should be able to specify options per ft index.

let's start with a few - ft_min/max_len, a language, and charset/collation.

this task is mostly about a framework for this, actual options will be added
later (e.g. a thesaurus as a part of WL#2428, stopword list as a part of WL#639,
relative column weights as a part of WL#25657)