Remove compat vsnprintf() (eggdrop requires posix 2001)

[michaelortmann]

Found by: vanosg
Patch by: michaelortmann
Fixes: #226

One-line summary:
Eggdrop depends on POSIX.1-2001, which defines vsnprintf().

Additional description (if needed):
This PR keeps egg_vsnprintf() for backward compatibility to (older) modules but redefines it to use vsnprintf() directly with a one-liner define in eggdrop.h.

All egg_vsnprintf() were replaced by vsnprintf():
find ./ -type f -exec sed -i 's/egg_vsnprintf(/vsnprintf(/g' {} \;
Note: don't do this in git directory. cd src/ first!

Test cases demonstrating functionality (if applicable):
No functional change. I tested 2 share bots, which successfully started and shared userfile.

[thommey]

AI finding:

P2 — The retained snprintf fallback loses its varargs definitions. src/compat/snprintf.c:85–107 still uses HAVE_STDARGS and VA_*, whose definitions this PR deletes. Configure automatically enables BROKEN_SNPRINTF on OSF/1, selecting this fallback even when libc provides snprintf. That path now cannot compile. Preserve the fallback’s standard varargs setup, or explicitly retire the fallback and its configure selection together.

[thommey]

After being told we require C99 and POSIX.2001, AI withdraws its finding and apologizes.

lgtm