[#640,!351] Access port from value given on the command line

[#640,!351] Access port from value given on the command line

The "-p" command-line switch (a debug option) tells Kea what port
to listen on.  This is easily accessible by the Fuzz object, so
get the port from this instead of an environment variable (which
had to be equal to the value given to the -p switch anyway.)