(Also link the dbus machine-id file to the one systemd will generate on
the next boot.  This seems safe and correct.)

Certain things (like systemd's dhcp client) use the machine-id as a seed
for derived values.  For instance, systemd's dhcp client offers a
ClientIdentifier in the new client style, and some servers will return
the same address to *all* requesting clients, instead of returning only
based on source MAC.  Can't have any of that confusion.