add: Detect when xattrs or perhaps ACLs prevent locking down a file's content

add: Detect when xattrs or perhaps ACLs prevent locking down a file's content

And fail with an informative message.

I don't think ACLs can prevent removing the write bit, but I'm not sure,
so kept it mentioning them as a possibility.

Should git-annex lock also check if the write bits are able to be removed?
Maybe, but the case I know about with xattrs involves cp -a copying NFS
xattrs, and it's the copy of the file that is the problem. So when locking
a file, I guess it will not be the copy.

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