fts: Renamed filter and tokenizer specific setting keys.

fts: Renamed filter and tokenizer specific setting keys.
Having the plural in the name didn't make a lot of sense, so all the
settings are now:

 fts_filters = name1 name2
 fts_filters_<lang> = name3 name 4
 fts_filter_<name> = key1=value1
 fts_filter_<lang>_<name> = key2=value2

 fts_tokenizers = name1 name2
 fts_tokenizer_<name> = key1=value1

So this also removes the confusion in settings if there existed a filter
with one of the language names.