[9fans] modest dns fixes

[9fans] modest dns fixes

[erik quanstrom]

i've pushed my modest dns fixes to sources in contrib quanstro/ndb.
this code has been running on my machine for a while, and i'm just
experimenting with it in more production environments.

in addition to the changes outlined before, i added some code to
terminate queries when no namserver is available.  this prevents an
O(n²) lookup, where n is the number of name servers.  (example
ndb/dnsdebug www.tridentaudio.co.uk; get some popcorn.)

i have specificly not addressed the database corruption problems.
these would seem at least partially to be a result of rrcopy() being
a shallow copy, and not doing anything about refcounting
subordinate or ->next records.  (thus more locking would seem
not the answer.)

nor have i addressed the bio problem.  making the buffer in
<ndb.h> 8k does sidestep the issue.

i hope this is useful, at least as a stopgap!

- erik