This adds the suit_worker_try_prepare / suit_worker_trigger_prepared
pair for using the SUIT worker, and breaks suit_handle_manifest_buf out
of suit_handle_url (where the latter now calls the former).
As part of factoring out reaping of the zombie worker thread, a locking
error that deadlocks the SUIT worker in case the race between the mutex
being unlocked and the thread being reaped hits the necessary handling
code is fixed (and mutex_unlock is called in the error path).
