Revert "API BREAK: change 'Agiodisc_t' output function from puts-alike to printf...

Revert "API BREAK: change 'Agiodisc_t' output function from puts-alike to printf-alike"

This reverts commit 4915673bd8826641c4227bb6e32e1d759f3b1df5. This also adds a
changelog entry for this.

Commit 4915673bd8826641c4227bb6e32e1d759f3b1df5 introduced a bug where
`gvprintf` was cast and stored into an incompatible pointer type, one returning
an `int` instead of `void`. On platforms like x86-64, this coincidentally works
out with no ill effects. But on platforms like Web Assembly this prevents using
plugins containing this code.