Feature: ipfs add/get --reflink

Although filestores are nice, they carry the issue of someone carelessly changing the file content and the cost of checks related to this problem. With a bit of filesystem magic, we can actually do a basically no-cost copy of data from a file to the blockstore or vice versa. The specific API is FICLONERANGE (Linux) and FSCTL_DUPLICATE_EXTENTS_TO_FILE (Windows).

A new boolean / string option `--reflink` should be added to `ipfs add` and `ipfs get`. It accepts three values: false (disable reflink), auto (enable reflink with fallback), and true (enable reflink no fallback).

- For `ipfs add`, `--reflink` only has an effect when the block store is a `flatfs`. `--nocopy` will disable `--reflink` since the block store isn't even used.
- For `ipfs get`, `--reflink` has an effect in two cases: when the block store is `flatfs` and when a block is being retrived from a `filestore`. The proposed `--nocopy` option (#6687) will work with this switch.

A big issue here is the datastore interface, as many bunches of `file:offset:length` information will need to be passed around for reflinking to happen in flatfs. (It *could* happen with other stores but no serious concurrent backend would expose these sort of internal data structure.) It might be easier for filestore since it already does that.