A number of tftp clients, including the one in Intel's pxe boot loader,
may intentionally stop a transfer using error code 0 (i.e., EUNDEF).
These are not real errors. Avoid spamming log files with these by
logging them at level LOG_DEBUG instead.
Discussed on -hackers with an initial patch proposal; this change is an
improved approach suggested by kan@.
9a2856b439