Chargeable-User-Identity (RFC 4372) in eapol_test

Implements the Chargable-User-Identity (CUI), as defined in RFC 4372.
Option "-i" causes the eapol_test to send a NUL CUI - which is a request to
send a CUI back. Capital "-I" allows to specify the value of the CUI.
This has been defined for cases where the client wants to reauthenticate.

src/radius/radius.h

@@ -87,6 +87,7 @@ enum { RADIUS_ATTR_USER_NAME = 1,
        RADIUS_ATTR_MESSAGE_AUTHENTICATOR = 80,
        RADIUS_ATTR_TUNNEL_PRIVATE_GROUP_ID = 81,
        RADIUS_ATTR_ACCT_INTERIM_INTERVAL = 85,
+       RADIUS_ATTR_CHARGEABLE_USER_IDENTITY = 89,
        RADIUS_ATTR_NAS_IPV6_ADDRESS = 95
 };