Fix bug introduced by #763 related to offline keys

Fix bug introduced by #763 related to offline keys

In some cases we want to keep expired signatures. For example, if the
KSK is offline, we don't want to fall back to signing with the ZSK.
We could remove the signatures, but in any case we end up with a broken
zone.

The change made for GL #763 prevented the behavior to sign the DNSKEY
RRset with the ZSK if the KSK was offline (and signatures were expired).

The change causes the definition of "having both keys": if one key is
offline, we still consider having both keys, so we don't fallback
signing with the ZSK if KSK is offline.

That change also works the other way, if the ZSK is offline, we don't
fallback signing with the KSK.

This commit fixes that, so we only fallback signing zone RRsets with
the KSK, not signing key RRsets with the ZSK.