Adopt style(9) using clang-format

[fel1x-developer]

I find atk's coding style doesn't follow some conventions, such as having space in <> or putting pointer * or reference symbol & with variable types, not variable names.

FreeBSD's coding style style(9) can be applied here. One main exception in style(9) opposed to the "convention" is that style(9) use 8-space indentation while most projects use 4-space indentation. (2-space indentation is predominant in web dev). We can discuss whether or not to use 8-space indentation if needed.