A generic replacement for src/net with some advantages:
- Shell script is well-suited to these kinds of tasks, making this
implementation much simpler.
- Allow arbitrary number of actions (not just init/exit), this can be
used to implement things like `dagger exec up/down`.
- Allow arbitrary number of scripts for each action/node tuple. This
will let us handle more complex scenarios in confd without having to
fiddle about with src/net.