CBM stdin Console RETURN-key Echo

When Commodore consoles were used through `stdin`, they echoed every key except the `RETURN` key.  But, the `stdin` consoles on other platforms usually do echo their `RETURN`/`ENTER` keys.  We want CC65's libraries to be consistent with each other.  And, we want them to be reasonably similar to other platforms (that aren't supported by CC65).

Therefore, this pull request makes CBM `stdin` echo `RETURN` too.