Automatic Termination After Background Job Completes

Setting an emacs/vi flag when starting dash, and using an external
command for a background job, the shell terminates once the job
completes.

1) Good (builtin command)
$ exec dash -E
  $ (printf "hello) &

2) Good (external command)
$ exec dash
  $ sleep 5 &

3) Bad (external command)
$ exec dash -E
  $ sleep 5 &