Change default TTLsig to one week

Change default TTLsig to one week

Commit dc6dafdad1bce4c59bec0dbc355650c384cfc4d9 allows larger TTL values
in zones that go insecure, and ignores the maximum zone TTL.

This means that if you use TTL values larger than 1 day in your zone,
your zone runs the risk of going bogus before it moves safely to
insecure.

Most resolvers by default cap the maximum TTL that they cache RRsets,
at one day (Unbound, Knot, PowerDNS) so that is fine. However, BIND 9's
default is one week.

Change the default TTLsig to one week, so that also for BIND 9
resolvers in the default cases responses for zones that are going
insecure will not be evaluated as bogus.

This change does mean that when unsigning your zone, it will take six
days longer to safely go insecure, regardless of what TTL values you
use in the zone.

(cherry picked from commit 32686beabc55727fc56e6fd02953233bf0daa8bd)