As a side benefit, this means that dbus-launch now understands
/etc/machine-id and not just /var/lib/dbus/machine-id.

Since machine_uuid comes out of libdbus allocated with dbus_malloc,
to avoid having to copy it from malloc-allocated to
dbus_malloc-allocated storage, it makes sense to change it to be
consistently dbus_malloc-allocated (particularly now that Bug #83115
has made use of internal symbols relatively painless). However, I'm
deliberately not changing the allocation model of any other strings
in dbus-launch right now; that's a larger yak-shaving exercise.