s4:gensec/spnego: only look at the optimistic token if we support the first mech

s4:gensec/spnego: only look at the optimistic token if we support the first mech

As a server only try the mechs the client proposed
and only call gensec_update() with the optimistic token
for the first mech in the list.

If the server doesn't support the first mech we pick the
first one in the clients list we also support.
That's how w2k8r2 works.

metze