eapol_test: Allow client IP address to be specified

eapol_test: Allow client IP address to be specified

Allow the user to set the IP address of the eapol_test client. This if
very useful when you have a machine with many interfaces and want to use a
particular one for testing RADIUS connectivity. For instance when I run the
national eduroam proxy I can only connect to other server from a particular
address, an our machine happens to have several IPs. So if I want to run
connectivity tests, I must make sure that my test uses a particular
interface. The -A option allows one to set this).

(jm: cleaned up to use radius configuration structure instead of global
variable for the address and added IPv6 support)