Support NetBSD 10+ version of struct statvfs
Description
In https://github.com/rust-lang/libc/pull/4265#issuecomment-2664275666 it was suggested to update the NetBSD struct statvfs code for changes in NetBSD 10.
See https://mail-index.netbsd.org/source-changes/2019/09/22/msg109266.html
I tested this code on NetBSD 10.99.12/x86_64 with bespoke test programs for fstatvfs, getmntinfo, getvfsstat, and statvfs.
This code is not ready to merge yet, since it doesn't work on NetBSD 9, and I need some guidance on how to handle this.
Should we add versioning support like freebsd12 etc. or is some other method preferred?
r? @tgross35
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