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.