Escape non-utf8 characters also from davix log messages

I don't trust data in log messages coming from Davix, because with non-utf8 characters this breaks logging done via python bindings. It is safer to apply utf-8 normalization (+ non-printable characters) not just to error messages but also to log messages from davix. I'm trying to make gfal2-util python3 compatible and this doesn't work because of davix bug that feeds logging gfal with garbage.