Implements 6 os/posix constants missing for macos

[ecpost]

Implements 6 os/posix constants missing for macos:

• SEEK_DATA and SEEK_HOLE already exist in posix.rs for a few other target_os's, but they are re-exported from libc, which does not currently include them for macos. So the the consts in this PR are hard-coded.
• O_EVTONLY, O_FSYNC, O_SYMLINK, and O_NOFOLLOW_ANY did not yet exist in posix.rs. Per the CPython docs, these are only available in macOS. 2 of the 4 are not available in libc. For consistency, 4 all are hard-coded in this PR.

I suppose it would be better to send a PR to libc to have these added (and I'm happy to do that as well), rather than hard-coding them here. But I don't know how long it would take for that change (in whatever libc version) to make its way back here. So I did them as constants that can be switched to libc re-exports if/when they're available in the future.

Ref: #1175

[youknowone]

Thank you! libc is very responsive and they have frequent release schedules. Once you submit a proper patch, it will be available in 2 weeks in high chance.

[youknowone]

https://docs.rs/libc/latest/x86_64-apple-darwin/libc/constant.O_FSYNC.html

[youknowone]

https://docs.rs/libc/latest/x86_64-apple-darwin/libc/constant.O_SYMLINK.html

[youknowone]

libc update: rust-lang/libc#2886

[ecpost]

Thanks for submitting the PR to libc! I saw your reply last night before I went to bed and was going to do that myself in the morning -- I should've let you know last night, sorry.

For the 2 requested changes, did you just want me to go ahead and use libc for the 2 it has already? (I had purposely done them all as constants for consistency. But I can fix those 2. Just confirming that's what you meant.)

[youknowone]

I want to keep Python-independent things as much as possible outside of RustPython.

To be honest, if you are not planning to work on another issue depending on this PR, I want to make everything referring libc before merging - unless they take too long time.
If you want to merge it immediately, yes, I want to use libc as much as possible. I want to use libc for those 2 values.

[ecpost]

Yeah, that makes sense. Let's leave it open until libc has them all. (No rush on my side. Just wanted to do an easy first PR on the project.)

[youknowone]

Thank you for understanding! I will notify you when it done.

[youknowone]

The PR is merged. It will be released in the next version (probably 0.2.133)

[youknowone]

@ecpost libc is updated! it took a lot longer time than expected.

[youknowone]

Thank you!