Allow applications to handle KeyboardInterrupt

[agronholm]

Currently the behavior between trio vs asyncio/curio differs in how ctrl+c is handled. Trio raises KeyboardInterrupt in the main task while asyncio and curio just clean up the tasks and exit. It would be preferable to unify the behavior across all backends so that at least applications using anyio.run() as their entry point could take advantage of this.

For asyncio and curio, this probably requires setting up a signal handler and raising the KeyboardInterrupt exception there. This will be particularly tricky on asyncio which, unlike curio, does not allow specific exceptions to be directly raised on tasks. It may require hijacking the coroutine object of the main task, much like what I did in asyncio_extras.

[njsmith]

Note that Trio will probably switch to delivering some KeyboardInterrupts out of trio.run instead of inside the main task; see python-trio/trio#1537 and linked issues

[agronholm]

Ouch, what a complicated subject this is. I'd better defer any actions on this and just document the current status.