This began as a cleanup of a few startling bugs in resolver.c, but I encountered so many other endemic issues (IMO) along the way that I just started from the top and groomed everything on my way through.  Apologies if the style doesn't quite match the rest of the codebase, but I hope it's at least a little easier to follow.

The resolver is still architected the same as the original (id est, a group of threads, all feeding from a common input stream in turn, resolving their data and responding independently before repeating), but it strives to do the things it does a little more correctly.

I've probably screwed up portability somewhere, and it may now require a non-vintage compiler.


Major fixes:

	* locking.  original had none around its core data structures (!)
	* functions no longer harbor static buffers (!)
	* non-blocking IO is more resilient


Other changes:

	* uses modern socket framework
	* can cache successful lookups as well as failures
	* cache lookups are now less than linear
	* uses native stdio locking


Possible future improvements to consider:

	* track more internal data, dump stats on a signal
	* make the threadpool dynamically sized
	* use a different work-queue model entirely