[krbdev.mit.edu #6848] gss library minor status codes are not exposed

[Greg Hudson via RT]

Our plan here is to try to expose the status codes in a manner as similar 
as possible to Heimdal, so that we don't create gratuitous 
incompatibilities.  Heimdal installs a <gssapi/gkrb5_err.h> containing 
symbols with a GSS_KRB5_S_ prefix.

I haven't worked out the details of the build system logic for this.