Hi Clément and Ludo!

Here's a new patch.  It mainly tidies up some formatting, makes it
possible to run more than one version of the DHCP daemon at the same
time (e.g., for IPv4, IPv6, and IPv4 over IPv6), and adds a system test
to verify that the DHCPv4 daemon can start up without error.

I tried adding a test case for DHCPv6, but I ran into complications.
Specifically, I'm not sure how to give an IPv6 address and subnet to the
eth0 interface within the test VM using the static-networking-service.
The DHCPv6 daemon requires the interface to have an IPv6 subnet
configured, so it refuses to run on that interface.

For now, it's nice enough, I think, that we have a DHCPv4 system test!
Please let me know what you think.

-- 
Chris