Add RADIUS server support for identity selection hint (RFC 4284)

Add RADIUS server support for identity selection hint (RFC 4284)

Previously, only the delivery option 1 from RFC 4284
(EAP-Request/Identity from the AP) was supported. Now option 3
(subsequent EAP-Request/Identity from RADIUS server) can also be used
when hostapd is used as a RADIUS server. The eap_user file will need to
have a Phase 1 user entry pointing to Identity method in order for this
to happen (e.g., "* Identity" in the end of the file). The identity hint
is configured in the same was as for AP/Authenticator case (eap_message
in hostapd.conf).