Add a test case for algorithm rollover.  This is triggered by
changing the dnssec-policy.  A new nameserver ns6 is introduced
for tests related to dnssec-policy changes.

This requires a slight change in check_next_key_event to only
check the last occurrence.  Also, change the debug log message in
lib/dns/zone.c to deal with checks when no next scheduled key event
exists (and default to loadkeys interval 3600).