Revise sample code intro text

We're not actually going to have separate code for the anonymous case.
Import sample code as of revision 6649de77c8

Tweaked to remove the code specific to my environment and leave the
else clause under if 0.

Don't worry about the trailing newline before the ']]' which closes
the CDATA block; it will make future imports/syncs easier.
Another remnant of separate anonymous code

Don't put "generic" in the subsection heading.
Tweak references

Don't cite 2119 any more, as we don't use it.

4401 is only informational, not normative -- it's just an example
of functionality provided by a GSS security context.
Add (non-empty) Acknowledgements
Add Security Considerations
Note that we assume v2

In particular v1 can generate DUPLICATE_TOKEN for context negotiation
without setting any bits in the error sections of the major status
field.  There may be others; I did not do a comprehensive audit.