v4 given v6

[crawdad@fnal.gov (Matt Crawford)]

> Given a 6bone node's v6 address, what's the best way to find a v4 address
> for the node (if it has one)?

I would look up the PTR record in ip6.int, then look up the A record
for that name.  However, there are some obstacles:

	The ip6.int is often not well-maintained.  We knew about this
	problem from the git-go.

	Some people are naming their IPv6 nodes in a separate
	subdomain, maybe because their regular DNS servers don't
	support AAAA records.