Redmine #4716: Lots of fixes to dirname(), esp. on Windows.

These problems were fixed:
* AddSlash() always adds Unix slash when argument is empty.
* DeleteSlash() removes root slash.
* FirstFileSeparator() returns NULL on all relative Unix paths, even
  if it has a file separator.
* ChopLastNode() gives empty string when removing last directory
  component of a relative path. It should give '.'.
* ChopLastNode() does not handle Windows drive letters.
* IsAbsoluteFileName() does not handle absolute paths on Windows that
  don't start with a drive letter. Both types are considered absolute
  paths.

This also fixes a Windows issue where creating a directory in policy
would lead to an error message about not being able to chdir into a
directory with the name ''. This was due to the final root slash not
being preserved.
As you wish; I poke only because you're fixing problems that the tests didn't cover before, so improving the tests (this restructure would make it much easier to add more) might ensure we don't have to come back and revisit this code another day with yet more of the same.