[th/replace-systemd-utils-1] replace usage of systemd code with our own code

we fork systemd code, mainly to get DHCPv6 and LLDP clients.

But we also started using some of the other systemd helpers.

If we ever will drop the systemd fork (which we want, because maintaining the fork is high effort), we need to move away from those.

This branch does that for some of the uses. In practice, it just copies the used systemd into our source tree and NM-ifies is.