Add a new service that, when enabled, will execute run-parts on
`/cfg/user-scripts.d`.
This strikes a balance between two conflicting objectives:
1. There should be no implicit way to schedule arbitrary code
execution on the device, i.e. no default run-parts directory.
2. It is very useful to have a way of scheduling arbitrary code
execution on the device, e.g. being able to install a debug script
on a production image.
With this feature, we still meet (1), since the feature has to be
explicitly enabled in the startup-config; but we also fulfill (2),
since we can easily enable it when needed.