allow systest_cli processes to run arbitrary external programs

[hyperthunk]

Currently the systest_cli handler provides the ability to run and monitor

1. startup
2. shutdown

and no other cases are handled. It would be nice if a systest:interaction/2 call could trigger running a script (e.g., a control script or whatever) but we need to abstract out some of the details so they're not tied to the proc implementation, especially:

1. launch handling
2. monitoring
3. interaction handling (ironically)

These need to be unpicked from the systest_proc API so we can use them anywhere, after which we can modify systest_cli so it will potentially track more than just the start + stop instances.

[hyperthunk]

It probably makes sense to make this work dependent on issue #75