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

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>