Replace calls to g_memdup() with g_memdup2()

Replace calls to g_memdup() with g_memdup2()

g_memdup() is subject to an integer overflow on 64-bit machines if the
object being copied is larger than UINT_MAX bytes. I suspect none of
these objects can actually be that large in practice, but it's easier
to replace all the calls than it is to assess whether we need to
replace them.

A backport in libglnx is used on systems where GLib is older than 2.68.x.

Signed-off-by: Simon McVittie <smcv@collabora.com>