This is just fundamentally broken. SPNEGO (RFC4178) is a protocol which
allows client and server to negotiate the underlying mechanism which will
actually be used to authenticate. This is *often* Kerberos, and can also
be NTLM and other things. And to complicate matters, there are various
different OIDs which can be used to specify the Kerberos mechanism too.

A SPNEGO exchange will identify *which* GSSAPI mechanism is being used,
and will exchange GSSAPI tokens which are appropriate for that mechanism.

But this SPNEGO implementation just strips the incoming SPNEGO packet
and extracts the token, if any. And completely discards the information
about *which* mechanism is being used. Then we *assume* it was Kerberos,
and feed the token into gss_init_sec_context() with the default
mechanism (GSS_S_NO_OID for the mech_type argument).

Furthermore... broken as this code is, it was never even *used* for input
tokens anyway, because higher layers of curl would just bail out if the
server actually said anything *back* to us in the negotiation. We assume
that we send a single token to the server, and it accepts it. If the server
wants to continue the exchange (as is required for NTLM and for SPNEGO
to do anything useful), then curl was broken anyway.

So the only bit which actually did anything was the bit in
Curl_output_negotiate(), which always generates an *initial* SPNEGO
token saying "Hey, I support only the Kerberos mechanism and this is its
token".

You could have done that by manually just prefixing the Kerberos token
with the appropriate bytes, if you weren't going to do any proper SPNEGO
handling. There's no need for the FBOpenSSL library at all.

The sane way to do SPNEGO is just to *ask* the GSSAPI library to do
SPNEGO. That's what the 'mech_type' argument to gss_init_sec_context()
is for. And then it should all Just Work™.

That 'sane way' will be added in a subsequent patch, as will bug fixes
for our failure to handle any exchange other than a single outbound
token to the server which results in immediate success.