Consider only making fields optional if they were added after Linux 2.6

The docs say that this library intends to support Linux versions greater than 2.6. Would it make sense to only make fields optional if they were added after 2.6? Anything before 2.6 is pretty old, and from my experience with rust-psutil, there's a lack of documentation and a lot of missing features from kernels older than 2.6.

edit: This would be a breaking change, and would require some changes to struct parsing too, but would also make things a lot simpler.