MSVC: add simple (v)snprintf implementation

_snprintf is not C99's snprintf!

This simple implementation, unlike _snprintf,
always null-terminates and returns the expected
return value (in most cases).
Other C99 behaviors (like format specifiers) require
adding a complete snprintf implementation.
Do we want or need that?

The same applies to vsnprintf (aka _vsnprintf).
Merge pull request #2498 from cremno/msvc-snprintf

MSVC: add simple (v)snprintf implementation