create an "nsupdate" script from DNS zone file differences

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.