How do you determine if a name is a valid Internet domain/host?

[Chris Siebenmann <cks@sys.toronto.edu>]

 I don't want to know where it goes, I want to know if it's a valid
domain/host name, without assuming that everything with dots in it
is. Note that $(smtp ...) isn't the answer, because it returns false
if the name is valid but MX'd to you (and this is hard-coded in the
router's nameserver code, not done in .cf code, so I can't change
it).

 Motivation: I'm considering rewriting some gatewaying code to
do funny address tricks, but I don't want to rewrite addresses that
are already valid Internet addresses.

	- cks