get_key_nb() returns 0 for either no input or "Esc" input

[nathandunk]

If there was no key pressed, get_key_nb() returns 0. But pressing Enter returns the KEY_ESC enum, which is also 0. We could either:

• Change return for get_key_nb() and get_ch_nb() to be -1
• Change the 0 enum to be something like KEY_NO_INPUT so that they result in the same thing

[nathandunk]

And actually, after doing some testing, it looks like a bunch of the special characters are wrong. I think you said that it hadn't been tested, @epezent . If that is right, I can make some changes to the windows case based on my testing.