mbcs.patch fixes PyUnicode_DecodeMBCS():
 - only use flags=0 if errors="replace" on Windows >= Vista or if errors="ignore" on Windows < Vista
 - support any error handler
 - support any code page (but the code page is hardcoded to CP_ACP)

My patch always tries to decode in strict mode. On decode error: it decodes byte per byte, and call unicode_decode_call_errorhandler() on error.

TODO:

 - don't use insize=1 (decode byte per byte): it doesn't work with multibyte encodings (like UTF-8)
 - use final in decode_mbcs_errors(): a multibyte character may be splitted between two chunks of INT_MAX bytes
 - fix all FIXME
 - patch also PyUnicode_EncodeMBCS()
 - implement suggested Martin's optimizations?
 - MB_ERR_INVALID_CHARS is not supported by some code pages (e.g. UTF-7 code page)

Is it necessary to write a NUL character at the end? ("*out = 0;")

It would be nice to support any code page, and maybe support more options (e.g. MB_COMPOSITE, MB_PRECOMPOSED, MB_USEGLYPHCHARS to decode).

It is possible to test different code pages by changing the hardcoded code_page value in PyUnicode_DecodeMBCS. Change your region in the control panel if you would like to change the Windows ANSI code page. You can also play with SetThreadLocale() and CP_THREAD_ACP to test the ANSI code page of the current thread.