chown, chgrp: decide --from on the file that is changed - #15029

--from was checked on a stat of the name, and the change was then made
by name, so a rename landing in between changed a file that never had
the required owner. Under -R -H a symlink met in the tree was also
checked itself while the change went to the file it points to.

Hold the file open and change it through that descriptor: O_PATH and
fchownat(AT_EMPTY_PATH) on Linux and FreeBSD, a read-only open and
fchown elsewhere, with O_SYMLINK for -h on macOS. A file found to have
been replaced after passing --from is left alone; as GNU does, that
fails without a message, and -v reports the change that was not made.
For a -R entry under -P, the file now under the name is judged in its
place instead, as GNU does.

Where a file cannot be held that way (a socket or device node, a
symlink under -h, or a file the caller cannot read), an unprivileged
caller, which can only change its own files, still changes it by name;
root gets an error for it.

The descriptor-based -R walk now runs on every platform safe_traversal
supports instead of only Linux, and a directory operand that cannot be
read is no longer skipped.