libdnssec: ignore reserved bits in DNSKEY flags
Otherwise Knot Resolver would be breaking a MUST at the very end of https://datatracker.ietf.org/doc/html/rfc4034#section-2.1.1
Real-life example: https://mailarchive.ietf.org/arch/msg/dd/W4lOTgd8-3NF0pbjtqP7YUz6Puk/