create an "nsupdate" script from DNS zone file differences
Description
The nsdiff program examines the old and new versions of a DNS zone, and
outputs the differences as a script for use by BIND's nsupdate program.
It provides a bridge between static zone files and dynamic updates.
The nspatch script is a wrapper around `nsdiff | nsupdate` that checks
and reports errors in a manner suitable for running from cron.
The nsvi script makes it easy to edit a dynamic zone.
