Exporting gssapi context, take two

[Sam Hartman]

In a side discussion with Nico, he indicated that he wants the
mechanism not to depend on special interactions with the glue layer.

We may have incompatible requirements.  I'll send out a message later
today exploring the space of possible options.

I assert axiomatically that we need to support mechanism specific
extensions in a manner that is easy for application authors to use.
Nico requires that the API between the mechanism and the glue layer be
GSSAPI.

Let's see if we can find a way of meeting both of these goals.