Add option to not send a Negotiate headers

If negotiation was attempted but failed do not send a new Negotiate header.
Useful when only one single sign on mechanism is allowed and to avoid
misleading login prompts in some browsers.

Added a test of the GssapiDontReauth option to the test suite.
Also added SPNEGO no auth test.

[SS: reworded and fixed commit subject/comment]
[SS: fixed whitespace errors and 80 column wrappings]

Reviewed-by: Simo Sorce <simo@redhat.com>
Close #65