dns zones for alias servers are not kept in sync with parent

when you create an alias server, it clones the parent DNS zone and replaces all record identifiers with the alias name instead.

however, if you then edit the parent server via the virtualmin dns editor OR the webmin bind server dns editor, then the changes are NOT replicated to the alias server zones.

this needs fixing so that whenever a dns zone is changed, the clone/duplication process is re-done over to the alias server so that it's kept in sync.

do it as follows:

- on virtualmin or webmin dns edit: instantly re-clone the zone
- run a cron job which looks at the file modified time of all parent dns zones and compares it to
   their alias zones, and re-clones the alias dns zones if the mtime isn't IDENTICAL
- after the cloning (regardless of whether it happened through the GUI or the cron job), simply use
   "touch" or the respective perl module to set the alias-zone's mtime to exactly match the
    parent zone's mtime

the latter cron job is important too, since some people manually edit zone files for various reasons, but it's okay if that check is only run every 10 minutes or something.

the cron job only needs to be created if the machine contains 1 or more alias domains.