Sanity check: GSSAPI SPI simplifications

[Sam Hartman]

It seems reasonable to make these changes only if we never plan to
decouple the krb5 mechanism from our mech glue into a separate DSO.

Put slightly differently, it seems reasonable for us to do this now.  It
would not be reasonable for a third party to take the same approach
because it would create problems if their mechanism were used with a
different mech glue.

In order to do this in a general manner, you'd want to standardize some
behaviors.