resolved: rework dnssec validation results

resolved: rework dnssec validation results

This adds a new validation result DNSSEC_UNSUPPORTED_ALGORITHM which is
returned when we encounter an unsupported crypto algorithm when trying
to validate RRSIG/DNSKEY combinations. Previously we'd return ENOTSUPP
in this case, but it's better to consider this a non-error DNSSEC
validation result, since our reaction to this case needs to be more like
in cases such as expired or missing keys: we need to keep continue
validation looking for another RRSIG/DNSKEY combination that works
better for us.

This also reworks how dnssec_validate_rrsig_search() propagates errors
from dnssec_validate_rrsig(). Previously, errors such as unsupported
algorithms or expired signatures would not be propagated, but simply be
returned as "missing-key".