This will replace the existing `g_memdup()` function for use within
GLib. It has an unavoidable security flaw of taking its `byte_size`
argument as a `guint` rather than as a `gsize`. Most callers will
expect it to be a `gsize`, and may pass in large values which could
silently be truncated, resulting in an undersize allocation compared
to what the caller expects.

This could lead to a classic buffer overflow vulnerability for many
callers of `g_memdup()`.

`g_memdup2()`, in comparison, takes its `byte_size` as a `gsize`.

Spotted by Kevin Backhouse of GHSL.

In GLib 2.68, `g_memdup2()` will be a new public API. In this version
for backport to older stable releases, it’s a new `static inline` API
in a private header, so that use of `g_memdup()` within GLib can be
fixed without adding a new API in a stable release series.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: GHSL-2021-045
Helps: #2319