Simplify and fix k5_check_cert_address

Get rid of the address union.  Store the result of get_cert_cn in a
signed variable so we can meaningfully check for negative results.
Make get_cert_cn return int for consistency with
X509_NAME_get_text_by_NID and its two callers.

Also add an emacs mode line to the top of the file.

ticket: 7929