The worker thread used for BackgroundAction was going to sleep for
1 second at a time (when there was nothing to do.) This made using
background actions for anything interactive quite unresponsive since
you had to wait up to 1 second before it even started on your task.
We now use a simple Unix pipe to signal the worker thread that a new
work item is available.
This makes Assistant way more responsive when typing. :^)
Sometimes you don't care about `joining()` the result of a thread. The
underlying pthread implementation already existed for detaching and
now we expose it to the higher level API.
