EZP-22191: As a User I expect API's with language filters to respect Always available flag

https://jira.ez.no/browse/EZP-22191

Different approach than https://github.com/ezsystems/ezpublish-kernel/pull/917, taking care of proposal from @lolautruche.

Todo:
- [x] 5.4/2014.09: Change default \$matchAlwaysAvailable value of LanguageCode Criterion to true
- [x] 5.4/2014.09/5.3.3: Introduce new \$useAlwaysAvailable argument for ContentService methods allowing language filtering
- [x] 5.4/2014.09: Change default value of  \$useAlwaysAvailable to true
- [x] 5.4/2014.09/5.3.3: Update Search service to pass \$fieldFilters 'languages' and 'useAlwaysAvailable' to ContentService->load()
- [x] Fix Solr failure (scoring changes)
Use `isset()` instead, as `false` will be considered as *empty*.