networkd: add dhcp server support

networkd: add dhcp server support

When enabled in [Network] it will set up a dhcp server on the interface, listening
on one of its statically configured IPv4 addresses and with a fixed size pool of
leases determined from it.

Example:

[Match]
Name=ve-arch-tree

[Network]
Address=192.168.12.5/24
DHCPServer=yes

[Route]
Gateway=192.168.12.5
Destination=192.168.12.0/24

In this case we will configure ve-arch-tree with the address 192.168.12.5 and
hand out addresses in the range 192.168.12.6 - 192.168.12.38.

In the future, we should (as suggested by Lennart) introduce a syntax to pick the
server address automatically.