CI: enforce linting C code in pmaports.git

It would be nice if we had a standard way of formatting C code, just like we have with PEP-8 for the python code (used in both pmaports.git and pmbootstrap.git so far).

TODO:

• find a good formatting/linting tool (clang?)
• write CI test that runs some C code formatting/linting tool and exits with a meaningful error in case something needs to be formatted
• create a merge request with that state, to show how it looks like when it fails
• create a separate commit in the same MR that fixes all the existing C code in pmaports.git (should only be a hand full of files)
• CI must pass now, merge request is ready to get merged

EDIT: I think something close to kernel code style guidelines would be nice, and fitting to the mainlining work that some of us are doing.