Add formatter (clang-format) configuration

[septatrix]

The project currently does not use consistent formatting throughout the codebase. This makes it sometimes confusing to know which pattern to follow. However, more importantly, in the context of the xv6 course/labs, this makes it harder to track changes and impossible to use an auto-formatter.

In many places there is trailing whitespace like spaces. Even without an explicit formatter, most IDEs will trim those. When checking ones changes before submitting this makes it hard to see which changes one implemented manually and in which places there were only whitespace changes. (I know that whitespace changes can often be suppressed but not everyone might know that, and it might not always be desired.) And using an auto-formatter is impossible because it will reformat most of the codebase making it impossible to track changes.

I propose to add a .clang-format file to the repository root and reformat the codebase accordingly. Additionally, CI should be set up such that a PR should only be accepted if it follows the code style to ensure future conformance.

[4g4]

Maybe it's not easy to do this. If we use formatter, then the line number reference to the codebase in the xv6 book will need to be updated accordingly. This needs amount of work I guess.

[septatrix]

Accepting any pull request which adds or remove a line would already require that. Running a formatter over the whole codebase would no doubt result in many changes but it is a one-time thing at least.

Independent of this it might make sense to automatically generate those references. The simplest way would be a marker comment (e.g. // MARKER ref:struct superblock), or alternatively something like ctags or clang-query. That way one script could generate an index file which one could \input in the tex source and have all references available.