clang-format all C code, enforce formatting in CI.

Description

This branch builds on the already checked-in .clang-format, making it easier to use locally and enforcing that C code in-repo is always formatted using it w/ a CI check.

I haven't attempted to extend the Windows Makefile with similar functionality on the assumption that all development is happening on Unix-like-systems and not Windows.

Resolves https://github.com/rustls/rustls-ffi/issues/327

Makefile: rm empty space.

Simple tidy commit.

Makefile: fix PHONY targets.

The all, clean, and test aren't traditional file targets: they don't produce a built artifact on disk from other files. If there were to be a file with a matching name found in the workdir these targets wouldn't perform their jobs as expected, thinking the workdir was up-to-date.

This commit marks these targets as "PHONY" targets, telling Make that they're not associated with produced files and are meta-targets for running helpful commands.

Makefile: add format-check and format targets.

This commit adds two new PHONY targets:

make format-check for enforcing that all .c/.h files are correctly formatted. This will be run in CI in subsequent commits once it will pass.
make format for reformatting all .c/.h files according to the clang-format profile. This can be run by developers to format code.

Both targets use find to locate all suitable .h and .c files. Crucially we configure find to ignore the target build directory, as well as the src/rustls.h file that is generated by cbindgen.

tests: universally apply clang-format.

This commit uses the new make format target to re-format all of the .c/.h files using clang-format. Afterwards the make format-check passes, where previously it flagged the re-formatted files in this commit as needing formatting.

ci: enforce C formatting w/ make format-check.

This commit extends the existing format task in CI that was enforcing the use of cargo fmt to also call make format-check to enforce the use of clang-format for C code.

rustls / rustls-ffi