lib-fts: Changed fts_tokenizer_next/final() to return error string. The current tokenizers can't fail, but if we're doing tokenization via external services they could fail.

lib-fts: Added fts_tokenizer_reset()

fts: Change tokenizer API to be able to return errors Modify fts_tokenizer_next() to return integer status codes. It returns 1 if a token was returned in *token_r, 0 if more input is needed and -1 on error.

Initial import for lib-fts. Parts of what this code does was already implemented internally by fts-lucene. lib-fts is intended to be usable for all the FTS backends. The APIs are still going to change a bit, but hopefully not after v2.2.17 release. Mostly written by Teemu Huovila.