user-record-nss: check if strings from pwd/spwd/grp/sgrp are valid utf-8

strv_extend_strv_utf8_only() uses a temporary buffer to make the implementation
conscise. Otherwise we'd have to rewrite all of strv_extend_strv() which didn't
seem worth the trouble for this one use outside of a hot path.

If the data is not serializable, we just pretend it doesn't exists.
This fixes #16683 and https://bugs.gentoo.org/735072 in a second way.