Work around Microsoft's broken vsnprintf implementation

Work around Microsoft's broken vsnprintf implementation

Microsoft's documentation says it has supported %zu (C99 standard format
for size_t) for some years but trying to use it causes crashes at least
on older versions of Windows.

Makes dpsnprintf and dpvsnprintf documentation more accessible (these
funcs have different return behaviour than the C standard ones).

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>