Safely shut down extension hooks.
It was possible to trigger shutting down an extension twice, which could

easily break hooks. Hooks assume they'll be initialized once and shut

down once, but this would break if extensions ever called shutdown on

themselves.
We now check an initialization state on the hook just to be sure before

we do any shutdown work.
Extensions wishing to shut down the hooks should now call

shutdown_hooks() instead of shutdown().

Unit tests pass.