Change type of `offset` arg in `H5Pset_external` to `HDoff_t`

[phil-opp]

Describe your changes

The off_t type is only 32-bit on Windows, which makes it impossible to link to higher offsets in large files.

The H5O_efl_entry_t struct defines its offset field already as HDoff_t, so no additional conversion is needed.,

Issue ticket number (GitHub or JIRA)

https://github.com/HDFGroup/hdf5/issues/3506

Checklist before requesting a review

• [ ] My code conforms to the guidelines in CONTRIBUTING.md
• [ ] I made an entry in release_docs/RELEASE.txt (bug fixes, new features)
• [ ] I added a test (bug fixes, new features)

[mkitti]

I think we might need to version the function in order to change the API like this.

[derobins]

Give us a little more time with this. We're thinking about ditching off_t entirely and moving to an unsigned integer.

[phil-opp]

Sure, sounds good!

[derobins]

Update: I'm starting to go through the library to make sure HDoff_t is used throughout. I think there's a few other internal changes we should also make. We have a lot of projects in motion right now, but I'll try to get the underlying work done in October so we can make this change.

[derobins]

Update: The internal changes should be complete now. We're discussing how versioning is going to work for this.

[derobins]

This can probably go in at this point, but we still use off_t in places like the tools and tests. Those will have to get fixed in the near future.

[derobins]

FWIW, this API call will NOT be versioned. If we didn't version API calls that use hid_t when we updated that from int to int64, we don't need to version this one.