copyfile: add an optional parameter to copy other stat data
Contrary to the comment, I didn't see any evidence that we were copying
atime/mtime at all. This adds a parameter to copyfile to optionally copy it and
other stat data, with the default being to not copy it.
Many systems don't support changing the timestamp of a symlink, but we don't
need that in general anyway -- copystat is mostly useful for editors, most of
which will dereference symlinks anyway.
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