Improve Kea test capabilities

Kea accepts the "-p" switch to change the port on which it listens. However, there appears to be no way to alter the port to which it sends replies. Similarly perfdhcp accepts the "-L" switch to alter the local port (the port on which it listens for responses? - this is not clear), but there appears to be no way to alter the port to which it sends packets.

Although full testing on the privileged ports using multiple systems would still need to be carried out before release, it would simplify a lot of development testing if Kea and perfdhcp could (with suitable switch settings) communicate via unprivileged ports on the loopback interface.