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.
Also: is `assert_equal(FileSeparator(in[i]), in[i] + off[i])` an option ?
I don't know what `assert_*` functions exist, but one might hope for this to produce a more informative failure message, if it fails.