Use the new UTF conversion functions in the last file that still
includes a private APR header. This is the almost final step towards
elimintaing those headers altogether.

* subversion/libsvn_subr/io.c: Include svn_utf_private.h
  (io_utf8_to_unicode_path, io_unicode_to_utf8_path): Reimplement, using
   the private Windoes-specific UTF conversion functions, and return
   an svn_error_t* instead of an apr_status_t.
  (io_win_file_attrs_set): Return an svn_error_t* and adjust for the
   changed signature of io_utf8_to_unicode_path.
  (io_win_read_link): Adjust for the changed signature of
   io_unicode_to_utf8_path.
  (svn_io_set_file_read_write_carefully): Adjust for the changed
   return type of io_win_file_attrs_set.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1534960 13f79535-47bb-0310-9956-ffa450edef68