Do not claim that it is safe to call gss_release_buffer twice

There is not actually anything in the spec that mandates this, and
though the well-known implementations do handle this, there may
be implementations that do not.  Fully compliant code must therefore
do its own checking to avoid double-frees and such.
Simplify a couple conditionals, per ghudson
Add a sample check_authorization function

Hardcodes a single authorized export name, and checks against it.