[9fans] dns, cs, and smtp

[David Presotto <presotto@closedmind.org>]

I updated dns, cs, and smtp to better handle the situation
of some chunk of the dns tree being unreachable.  Before the
changes, smtp would often return mail prematurely if we
couldn't reach part of the dns tree.  Now it just leaves the
mail queued to try again later.

A side effect of this change is to make cs and dns return
better error messages.

I also fixed a problem with inherited dns domain's.  When I
added caching of negative replies, I broke the handling of
inherited dns domain's.  For example, if you had in your /lib/ndb/local
file something like:

ipnet=mh-astro ip=135.104.0.0
	dnsdomain=cs.bell-labs.com
	dnsdomain=research.bell-labs.com
	dnsdomain=x.bell-labs.com
	dnsdomain=bell-labs.com
	...

any attempt to access an undistiguished domain name 'x' should
have tried x.cs.bell-labs.com, then x.research.bell-labs.com,
then x.x.bell-labs.com, then x.bell-labs.com.  Unfortunately,
when I added the negative cached entries, the first domain that
returned a negative reply would cause the search to stop.  Its
fixed now.  It was broken for over a year and I don't recall
anyone complaining.  I just happened to notice it this weekend
when I was testing the other stuff.

Also, I fixed a few nits in dnsdebug.

I've been running the code for a few days without ill effects here.

Tell me ASAP if you have any problems with it.