It was naive of me to think that KDirNotify was sufficient to watch all indexed folders.
Of course even fam has a pretty small number of maximal paths that can be watched.

Thus, now the strigi service uses an idea from Amarok: simply check all folders' modification time every 2 minutes and update those that changed.

Since that only catches new files and removed files but not changed files, we also update the whole index every half hour. That way we are sure to catch all changes, some with a delay. I don't think we can do any better ATM.