Heimdal build fixes

Heimdal build fixes

Heimdal's GSS-API uses constant context, credential and name handles in various
places (including some where the handle is actually mutable, such as a context
that is having its sequence state updated by a message protection operation).
Conditionally define the exported API to match this. We use gss_const_XXX
internally now which should not regress with MIT.

Note: we really need to look at how locking is done, I think in my initial
implementation I was probably a bit overzealous with the mutexes.