networkd: netdev - allow setting MACAddress in .netdev files

It may sometimes be necessary to specify the MAC address of a netdev.
Let us set the correct one from the get-go, rather than having the
kernel generate a random one, and then change it after.
Revert "Revert "networkd: netdev - set predictable mac address when creating netdev""

This reverts (and rewrites) commit 7d95c772cba1836545459760273b13f2e01dd2a8.

The issue blocking this feature has now been fixed in the kernel, and backported
to the various stable kernels.

Our netdevs will now have stable MAC addresses, even if one is not specified.