http: fix endless loop when handshake is rejected
In a mixed Kerberos/NTLM setup with SSO disabled but a valid TGT, a
wrong password caused an endless loop. When the final handshake was
rejected the context was already marked "ready" from our side. When in
the next round a NULL token was passed down, then sip-sec-gssapi.c
drops the ready context and restarts the authentication handshake
instead of returning a failure.
We now set a flag on the HTTP request so that a failed final handshake
is recognized and the request is aborted correctly.