Add a .clang-format, please

[carmiac]

I know that the BeOS formatting style is unique, and that there has been, um, passionate debate about that. I don't want to debate any of that. What would be nice is a clang-format that matches what is there currently as closely as possible, and a policy on how to handle whitespace changes. Should whitespace-only changes be submitted as a separate PR, then a followup? Or in the PR if there aren't too many? Or a one-time full repo formatting pass?

[Numerio]

I'm not against it, but since I'm literally in building mode, I can't really be too picky on that. My concern is that historically no automated tool was able to grab the haiku codestyle, and if anyone wanted to submit it, it should be a really in-depth .clang-format file, which will probably take weeks to be made and we're back to the starting point. But, before or later I planned to do a full repo-restyle. What do you mean by "handle whitespace changes"?