[th/nettools-n-dhcp4-as-internal-plugin] dhcp: switch IPv4 "internal" DHCP client to use "nettools" backend instead of "systemd"

Previously, our "internal" DHCPv4 client is based on a fork of systemd code. This manner of maintaining the fork is problematic. The solution is to use a proper library: n-dhcp4 from the nettools project.

We already have these two as undocumented plugins available, by setting either "dhcp=systemd" or "dhcp=nettools". This is only for testing. Users are only supposed to use the "internal" plugin.

Up until now, the "internal" DHCPv4 plugin was based on "systemd" code. Change that to use "nettools" instead.

Possibly this breaks something, and we need to fix it. But do this early so we have time to test the nettools plugin and identify issues.

For the user, this change should be entirely transparant.