Override console Ctrl+C event handler with handler in module signal

[BCSharp]

This is basically a fix for the signal module on POSIX. What was happening was that on POSIX, signals (to be more accurate: SIGINT only) are handled by SimpleSignalState, which installs a ConsoleCancelEventHandler on Console.CancelKeyPress. However, BasicConsole also installs its handler to handle Ctrl+C, and it happens very early during execution, way before module signal can be imported. Since the event handlers are executed in the order of registrations, BasicConsole is seeing the event before signal, calling ThreadAbort (which obviously does not work on .NET), and module signal had no chance to even react to SIGINT. This means that setting SIGINT to be ignored, or to terminate the program, or to call a custom event handler was ineffective.

This problem does not occur on Windows because NtSignalState installs its event handler by making a syscall to SetConsoleCtrlHandler in kernel32.dll, which does its callbacks in a "last registered — first called" fashion, so the late registration in signal overrules BasicConsole.

There is no straightforward way to prevent BasicConsole to register its event handler, but there is a way to override what it does by modifying a public property on BasicConsole, and this is what this PR does. The only quirk here was that the reference to a BasicConsole instance was not readily available to modules, so I added a new property on PythonContext to provide it. If there is a better way of doing it I would be glad to hear it.

[slozier]

Could do a #nullable enable / #nullable restore?