[krbdev.mit.edu #7592]

[Simo Sorce via RT]

In gss_import_sec_context there is a mismatch between the context
created and the mechanism assigned to the union context when an
interposer plugin is involved.

The context mech_type should always reflect the actual context type
stored in internal_ctx_id, however currently the wrong mech_type is saved.
When an interposer context is returned the mechanism type is set to that
of the real mechanism, and conversely when a real context is returned
the interposer mechanism type is stored.

The net effect is that the wrong mechanism context is cast and is passed
to subsequent functions.