Fortinet requires us to check for an HTTP error response only over TLS

If the Fortinet PPP connection request *succeeds* over TLS, there is no HTTP
response before we start exchanging PPP packets.  If it *fails*, there is an
HTTP response.

If the Fortinet PPP connection request is over DTLS, a 'svrhello' response
is expected regardless of whether it succeeded or failed. This is handled
by fortinet_dtls_catch_svrhello()

Let's only check for that HTTP response in Fortinet if we're definitely
connecting over TLS.  The "proceeding to tunnel stage" test in
'fortinet-auth-config-tests' verifies the correctness of the HTTP response
parsing behavior.

Fortinet connection response matrix ("Don't blame me, I didn't design this."):

           \ TRANSPORT
    STATUS  \             TLS               DTLS
             +            ---------------   -------------------
    Success  |            immediate → PPP   SVRHELLO 'ok' → PPP
    Failure  |            HTTP response     SVRHELLO 'fail'

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

fortinet.c

@@ -541,7 +541,7 @@ static int fortinet_configure(struct openconnect_info *vpninfo)
 	 * FortiOS 4 was the last version to send the legacy HTTP configuration.
 	 * FortiOS 5 and later send the current XML configuration.
 	 * We clearly do not need to support FortiOS 4 anymore.
-	 * 
+	 *
 	 * Yet we keep this code around in order to get a sanity check about
 	 * whether the SVPNCOOKIE is still valid/alive, until we are sure we've
 	 * worked out the weirdness with reconnects.
@@ -665,6 +665,7 @@ int fortinet_connect(struct openconnect_info *vpninfo)
 	 *
 	 * Don't blame me. I didn't design this.
 	 */
+	vpninfo->ppp->check_http_response = 1;
 
 	/* Trigger the first PPP negotiations and ensure the PPP state
 	 * is PPPS_ESTABLISH so that ppp_tcp_mainloop() knows we've started. */

ppp.c

@@ -258,6 +258,7 @@ int ppp_reset(struct openconnect_info *vpninfo)
 	ppp->ppp_state = PPPS_DEAD;
 	ppp->out_asyncmap = 0;
 	ppp->out_lcp_opts = BIT_MRU | BIT_MAGIC | BIT_PFCOMP | BIT_ACCOMP | BIT_MRU_COAX;
+	ppp->check_http_response = 0;
 
 	switch (ppp->encap) {
 	case PPP_ENCAP_F5:
@@ -268,7 +269,6 @@ int ppp_reset(struct openconnect_info *vpninfo)
 		/* XX: Fortinet server rejects asyncmap and header compression. Don't blame me. */
 		ppp->out_lcp_opts &= ~(BIT_PFCOMP | BIT_ACCOMP);
 		ppp->encap_len = 6;
-		ppp->check_http_response = 1;
 		break;
 
 	case PPP_ENCAP_F5_HDLC: