Some enhancements for OS/2 ap_canonical_filename:

- Log proper error message instead of error code on failure
- In case of error caused by an invalid file name, don't return an empty
  string as that tends to confuse things rather than make them better.
- Avoid using the very expensive ap_os_systemcase_canonical_filename()
  unless it's truely necessary.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86011 13f79535-47bb-0310-9956-ffa450edef68