win32: make vlc_vsnprintf more like c99 vsnprintf

win32: make vlc_vsnprintf more like c99 vsnprintf

Fixes following issues using MSVCRT _snprintf:
 - Failure to null terminate all strings
 - Failure to return number of characters that would've been
   printed had the buffer been sufficiently large
 - Failure to accept size = 0 (and str = NULL) to determine
   final output length.

NB, the third issue above is fixed on *some* win32 implementations,
however it is not officially documented as being so.

Signed-off-by: David Flynn <davidf@rd.bbc.co.uk>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>