allow to exit dune via signals (e.g. ^C) during the initial scan

[arbipher]

Desired Behavior

During the running dune build, dune can respond to ^D signal and exit the execution as soon.
Currently, it seems to ignore it until finish initial scanning, which is slow if the sub-directory contain dune-related files.

The issue is suggested by @jeremiedimino for reminding in this OCaml Discuss post.
He suggests: insert checkpoint to check for signals during the initial scan.

Pressing ^C several time quickly can perform an emergency exit (just knowing this), and I suggest:
removing dune-project file if there is no one before running dune build.

Example

The original topic in the post is on how dune should behave when there is no dune-project or dune-workspace. A pain point scenario is assuming you have directories where code is just a path for containing true dune projects.

code
   |-----pj1 (a dune project)
   |-----pj2 (another unrelated dune project)

Occasionally, you might run dune build at path code rather than at code\pj1. you realize it's a wrong action but dune is beginning initializing process.

That should be quite easy to do with the current refactorings we are doing. Adding this to Dune 3.0.

I believe this is fixed in main.

[rgrinberg]

Needs a CHANGES entry.