Unregister thread key in SPNEGO finalization

Unregister thread key in SPNEGO finalization

Commit d160bc733a3dbeb6d84f4e175234ff18738d9f66 (ticket 7045) added a
new thread key K5_KEY_GSS_SPNEGO_STATUS and registered it in SPNEGO
library initialization, but neglected to unregister it in
finalization.  As a result, loading, unloading, and reloading
libgssapi_krb5 could throw an assertion failure if libkrb5support
remained loaded.  Unregister the key in SPNEGO finalization and add a
test case.

Reported and investigated by Adam Dabrowski.

ticket: 8614
tags: pullup
target_version: 1.18-next
target_version: 1.17-next