Start separating protocol-specific methods from generic VPN support

It looks like we can understand the Juniper oNCP protocol and it fits fairly
well with the OpenConnect model — you first authenticate and are rewarded
with a cookie, and then you make the actual connection to the HTTPS server
and *can* transport data over that, or ideally you have a UDP transport
(in this case ESP) instead.

So start factoring out the methods which are specific to the VPN protocol,
with a view to making Juniper support possible too.

Starting with cstp_bye().

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

library.c

@@ -91,6 +91,8 @@ struct openconnect_info *openconnect_vpninfo_new(const char *useragent,
 	bindtextdomain("openconnect", LOCALEDIR);
 #endif
 
+	vpninfo->proto.vpn_close_session = cstp_bye;
+
 	return vpninfo;
 
 err:

mainloop.c

@@ -224,8 +224,8 @@ int openconnect_mainloop(struct openconnect_info *vpninfo,
 #endif
 	}
 
-	if (vpninfo->quit_reason)
-		cstp_bye(vpninfo, vpninfo->quit_reason);
+	if (vpninfo->quit_reason && vpninfo->proto.vpn_close_session)
+		vpninfo->proto.vpn_close_session(vpninfo, vpninfo->quit_reason);
 
 	os_shutdown_tun(vpninfo);
 	return ret < 0 ? ret : -EIO;

openconnect-internal.h

@@ -201,7 +201,13 @@ struct proxy_auth_state {
 	char *challenge;
 };
 
+struct vpn_proto {
+	int (*vpn_close_session)(struct openconnect_info *vpninfo, const char *reason);
+};
+
 struct openconnect_info {
+	struct vpn_proto proto;
+
 #ifdef HAVE_ICONV
 	iconv_t ic_legacy_to_utf8;
 	iconv_t ic_utf8_to_legacy;