Since it appeared with GLib 2.68.0, we could not change this until we
bumped the dependency which has only become possible a few days ago
(since Debian testing is our baseline for dependency bumps). Cf.
previous commit.

As this is a drop-in replacement (just a guint parameter changed to
gsize to avoid integer overflow), search-and-replace with:

> sed -i 's/g_memdup\>/g_memdup2/g' `grep -rIl 'g_memdup\>' *`

… followed by a few manual alignment tweaks when necessary.

This gets rid of the many deprecation warnings which we had lately when
building with a recent GLib version.