Remove outdated stuff (e.g. VS has snprintf() now).

I'd like to have a clean build (no warnings) on both VS and gcc.
Currently VS has a bunch of printf code warnings in
diagnostics.cpp, diagnostics_win.cpp, and stackwalker_win.cpp.
These should be easy to fix.
gcc has a bunch of sprintf buffer-size warnings, not easy to fix.
There are lots of warnings in zip (to be fixed with vcpkg?)
and in old graphics code.

Use snprintf() instead of sprintf() in a few places
(should eventually do it everywhere)

Notes:

(void)x;
seems to work (cross-platform) for suppressing unused arg warnings.

int n;
n = snprintf(buf, sizeof(buf), ....)
(void)n;

works in some cases for suppressing buffer-size warnings.
But not all cases.  Not sure why.