src: proposed coding style for C, Linux++

The intention of this is to serve as a help/guidance for both devs and
reviewers of pull requests.

Baseline is taken from Linux v6.5.6, with some (possibly) controversial
additions (at the end of the file):

 - Do not enforce max line length
 - Alignment of variables, both when assigning and declaring
 - Use spaces to adjust when leading tabs are not enough (as Emacs)

I've also added a couple of exceptions for macros like TAILQ_FOREACH()
which are used as control statments (if, while, for), so the opening
brace should be on the same line -- otherwise clang-format thinkts we
are creating a recursive function ...

Signed-off-by: Joachim Wiberg <[email protected]>