Simplify some Win32 specific code by using svn_subr__win32_utf16_to_utf8.

* subversion/libsvn_subr/config_win.c
  (includes): Add private/svn_subr_private.h.
  (svn_config__win_config_path): Use svn_subr__win32_utf16_to_utf8.
  (parse_section): Add comment.

* subversion/libsvn_subr/nls.c
  (includes): Add svn_subr_private.h.
  (svn_nls_init): Simplify win32 specific code. Assume Windows 2000
    or later. Report buffer overflows instead of using a truncated
    path.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1534083 13f79535-47bb-0310-9956-ffa450edef68