x/sys/unix: Possible breaking change with Pselect on linux

[mauri870]

Follow up to https://go-review.googlesource.com/c/sys/+/510195
Relates to #61251

Jesse Rittner words:

The Go Pselect wrapper never followed this requirement, which means this is a breaking change. 
I agree it was technically a bug, but it can break existing programs in hard-to-detect ways, 
particularly where EINTR is involved.

[ianlancetaylor]

Thanks, but I think that changing this is fine. We are not as strict about backward compatibility in the x/sys repo as we are in the standard library.