Don't abort Negotiate auth when the server has a response for us

It's wrong to assume that we can send a single SPNEGO packet which will
complete the authentication. It's a *negotiation* — the clue is in the
name. So make sure we handle responses from the server.

Curl_input_negotiate() will already handle bailing out if it thinks the
state is GSS_S_COMPLETE (or SEC_E_OK on Windows) and the server keeps
talking to us, so we should avoid endless loops that way.