Fix warning in building `scripts/src/killdisp.c`, ignore killdisp binary, makefile --> Makefile

This addresses 3 minor issues:

1. The C `main()` function expects to `return 0;` on success. Declaring it returning `void` technically works, but there's no reason not to use `int`, especially as `void` creates a warning when building in gcc.
2. The `scripts/killdisp` binary should not be part of the repository. I've added a rule in `.gitignore` to ignore it.
3. By convention, prefer `Makefile` over `makefile`.