Lighting thread and sync tweaks.

- Check the result of pthread_create() so we can detect errors starting
  the lighting data thread.
- pthread_detach() the lighting data thread so we don't run out of
  resources triggering an error we would have to detect in the new
  code above.
- Don't bother running the sync code if we're not outputting any
  lighting data.
Merge branch 'threaded' of github.com:FalconChristmas/fpp into threaded