MFC r231973: Avoid error log for transfer stop w/o error code.

MFC r231973: Avoid error log for transfer stop w/o error code.

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@.

git-svn-id: svn://svn.freebsd.org/base/stable/9@239432 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f