Ray suggested that error reporting could also be improved.  After I took
another look at the branch, I noticed two bugs it introduced that flew
under the radar.  Those are now fixed (details are described in commit
messages).  Ray's suggestion is also implemented.

In the process, I "ported" the -z switch from bin/tests/rdata_test.c
(testing zero-length wire form RDATA).  I will keep looking for
interesting bits in that file and report back when I am done.