resolved: fix mistaking QU bit for cache-flush bit

resolved: fix mistaking QU bit for cache-flush bit

RFC 6762 defines the top bit in RRs to mean cache flush (section 10.2),
and the top bit in questions to mean that a unicast reply is wanted
(section 5.4).

dns_packet_read_key() is used for parsing both questions and RRs.
When called from dns_packet_extract_question(), the top bit being set
should not result in the packet being rejected as invalid.

Fixes https://github.com/systemd/systemd/issues/17973

(cherry picked from commit 82d39576462ba66a4316307ce1ded97272e1245f)