Fix incompatbilities with MS-Windows 2000 and older

Fix incompatbilities with MS-Windows 2000 and older

* src/w32.c <multiByteToWideCharFlags>: New global variable.
(filename_to_utf16, filename_from_ansi, check_windows_init_file):
Use it instead of the literal MB_ERR_INVALID_CHARS.
(maybe_load_unicows_dll): Initialize multiByteToWideCharFlags as
appropriate for the underlying OS version.  For details, see
http://lists.gnu.org/archive/html/emacs-devel/2016-01/msg00835.html.
* src/w32.h: Declare multiByteToWideCharFlags.
* src/w32fns.c (Fx_file_dialog, Fw32_shell_execute)
(add_tray_notification): Use multiByteToWideCharFlags instead of
the literal MB_ERR_INVALID_CHARS.
(_resetstkoflw_proc): New typedef.
(w32_reset_stack_overflow_guard): Call _resetstkoflw via a
pointer, as this function is absent in msvcrt.dll shipped with W2K
and older systems.

Copyright-paperwork-exempt: yes