lib-fts: Added fts_language_register() to register more languages in plugins.

lib-fts: fts_language_list_init() API changed to return errors.

lib-fts: Assume that fts_language.name is always lowercased and has two letters. Anything else just unnecessarily complicates the code.

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.