[RFE] Improve etag handling with --etag-compare-update

Hello,
I'm trying to implement a "download only if updated on a remote
resource" functionality with a curl script using etags.

Currently the etag file saved with --etag-save becomes useless after a
single update on a remote resource. This can be worked around on the
client side by saving etag to a temporary file, comparing it with
previously stored etag file etc. But it would be nice to have a
--etag-compare-update <file> option that would work like --etag-compare
plus overwrite the <file> with the new etag if one was returned by
server.

Thanks.
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