A few more certificate extensions.
Let's not test zlib itself.
Backend: Fixed stale thread state in poll devices.

The current thread was left swapped in during co_wait_interpreter()
in POLL_DEVICE_SET_EVENTS().

Also fixes condition variable leak (probably not relevant in most
thread implementations, but...).
Backend: Support multiple concurrent threads in the backend.

This is likely to simplify code that uses multiple backends with
multiple threads (cf SSL).