Remove the sys member

Inline the sys member crate. Use the atty crate in main for broader support (windows, getting closer) and to avoid exposing the sys directly. Use stdlib for exec. Testing to remove the set_unique_pid method in main, since it was making quirk (receiving input even when backgrounded) and did not seem useful. Needing approval for the remove.

@Tommoa Can you check if it works on MacOS and if the error previously mentioned has disappeared?