Map `Ctrl+C` to cancellation of the main function

[badeend]

Currently, WASI does not describe any way for a CLI guest to receive the Ctrl+C signal (SIGINT on Unix). This signal is commonly used to request graceful shutdown, giving the application some time to e.g.:

• clean up resources,
• flush buffers to disk,
• stop listening for new network requests, while letting the existing ones run to completion,
• etc..

The component model has built-in support for async cancellation, and I think that in the CLI world Ctrl+C maps neatly to cancellation of the main (run) function.

I propose we update the documentation of the run function to say something along the lines of:

If a host wishes to propagate Ctrl+C into the guest, then cancellation of the run task is the canonical way to do so.

WDYT?

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Related: https://github.com/WebAssembly/WASI/issues/166