eap-radius: Add option to set interval for interim accounting updates

Any interval returned by the RADIUS server in the Access-Accept message
overrides the configured interval.  But it might be useful if RADIUS is
only used for accounting.

conf/plugins/eap-radius.opt

@@ -5,12 +5,16 @@ charon.plugins.eap-radius.accounting_close_on_timeout = yes
 	Close the IKE_SA if there is a timeout during interim RADIUS accounting
 	updates.
 
+charon.plugins.eap-radius.accounting_interval = 0
+	Interval for interim RADIUS accounting updates, if not specified by the
+	RADIUS server in the Access-Accept message.
+
 charon.plugins.eap-radius.accounting_requires_vip = no
 	If enabled, accounting is disabled unless an IKE_SA has at least one
 	virtual IP.
 
 charon.plugins.eap-radius.class_group = no
-	Use class attributes in RADIUS-Accept messages as group membership
+	Use class attributes in Access-Accept messages as group membership
 	information.
 
 	Use the _class_ attribute sent in the RADIUS-Accept message as group

src/libcharon/plugins/eap_radius/eap_radius_accounting.c

@@ -488,6 +488,16 @@ static void send_start(private_eap_radius_accounting_t *this, ike_sa_t *ike_sa)
 	message->add(message, RAT_ACCT_SESSION_ID,
 				 chunk_create(entry->sid, strlen(entry->sid)));
 
+	if (!entry->interim.interval)
+	{
+		entry->interim.interval = lib->settings->get_time(lib->settings,
+					"%s.plugins.eap-radius.accounting_interval", 0, lib->ns);
+		if (entry->interim.interval)
+		{
+			DBG1(DBG_CFG, "scheduling RADIUS Interim-Updates every %us",
+				 entry->interim.interval);
+		}
+	}
 	schedule_interim(this, entry);
 	this->mutex->unlock(this->mutex);