[Fud2 | Request] Cleanup the work directory on SIGINT

[EclecticGriffin]

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I run into this issue a bunch when I'm running runt tests and kill the execution with CTRL-C since doing so leaves all the work directories lying around. I am not certain that runt will propagate the SIGINT but fud2 should still probably react appropriately when it receives one.

[sampsyo]

Thanks; this is a good idea. I'm not 100% sure how to do this, but I think we probably want to handle SIGINT with signal-hook and somehow access some sort of mutable global state to find out if there is something to clean up. Then, we'll want to make sure that the Ninja subprocess also gets terminated when we then immediately exit.