Fix interrupt signal handling during shell.RunInteractiveShell()

[Andriamanitra]

closes #2612

[Andriamanitra]

This also slightly changes Ctrl-C behavior during the "Press enter to continue" prompt:

- Pressing Ctrl-C does not immediately interrupt the process
- The program will close after the user presses enter if Ctrl-C was pressed.
+ Pressing Ctrl-C does not interrupt the process
+ The program returns to the editor as normal after user presses enter.

I do not think this is an issue so I didn't bother with trying to find a workaround. Ctrl-C keybinding still works normally once the editor is restored.

EDIT: Actually this also affects users trying to kill micro from outside with SIGINT (for example kill -INT micro). I tried adding signal.Reset(os.Interrupt) but unfortunately that does not work due to a bug in golang: golang/go#46321

[Andriamanitra]

As explained in the issue comments I don't think this approach will work without messing up SIGINT handling ☹️