Do not fork and exec cmp(1); instead, simply fstat(2), mmap(2), and

Do not fork and exec cmp(1); instead, simply fstat(2), mmap(2), and
compare the files directly, allowing a much stricter pledge(2), at
very little cost: merely 15 additional lines of very simple code.
Suggested by George Brown <321 dot george at gmail dot com> on misc@.