Force full sign to generate new signatures

Force full sign to generate new signatures

When introducing the kasp logic, a full sign of the zone did not
generate new signatures for the new active keys during a ZSK rollover.

The introduced kasp logic ensured that the rollover is performed
smoothly, as in the signatures are only replaced if the old signature
is close to expiring (depending on the signatures-refresh option).

Fix by maintaining a fullsign boolean value in the signing structure,
that will ensure the RRsets are signed with the correct key, rather
than a similar good key.

In case of a fullsign, we can also remove signatures from inactive
keys.

Remove the unused dns_zone_signwithkey function.