Mark nearly all C/C++ code as under clang-format in .gitattributes,
and run clang-format over the entire code base.

The real committer is Steinar H. Gunderson <steinar.gunderson@oracle.com>,
but the author has been set to Anonymous Reformatter so that git blame won't
show myself as author of all existing code. To ignore whitespace changes caused
by this commit, please use git blame -w. For ignoring larger changes, you can
run blame on the commit immediately preceding this, as follows:

  git blame clang-format-base^ -- path/to/file.cc