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.