Add clang-format config file and re-format with clang-format 15

Add clang-format config file and re-format with clang-format 15

Add a clang-format config file, copied from:

  http://git.efficios.com/?p=babeltrace.git;a=blob;f=.clang-format;h=cba562531157c82aefa5a16f861ef044fa5388e0;hb=a165a5742a2b173e59900e515caa3b37a0afb42f

Adjust it slightly to remove the things that are only relevant in the
context of babeltrace.

Re-format all C files with clang-format 15.

Change-Id: If80c93c139d08fafaf11a8fd8dbca23fb80d4aca
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>