pass IDLE_TIMEOUT to configuration script

`vpninfo->idle_timeout` was added in
37fbeed, for use by the Java API and
Android ics-openconnect, which uses it to keep the connection alive at
appropriate intervals.

This patch makes this variable available to the normal vpnc-script used by
OpenConnect, so that it can likewise spawn a keepalive process if desired.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>

script.c

@@ -226,6 +226,11 @@ void prepare_script_env(struct openconnect_info *vpninfo)
 
 	script_setenv_int(vpninfo, "INTERNAL_IP4_MTU", vpninfo->ip_info.mtu);
 
+	if (vpninfo->idle_timeout)
+		script_setenv_int(vpninfo, "IDLE_TIMEOUT", vpninfo->idle_timeout);
+	else
+		script_setenv(vpninfo, "IDLE_TIMEOUT", NULL, 0, 0);
+
 	if (vpninfo->ip_info.addr) {
 		script_setenv(vpninfo, "INTERNAL_IP4_ADDRESS", vpninfo->ip_info.addr, 0, 0);
 		if (vpninfo->ip_info.netmask) {