x/sys/windows: Add SetFileValidData and SetFilePointerEx support

[kanak-clumio]

Currently, this package does not support SetFileValidData and SetFilePointerEx syscalls. Is there any specific reason not to have this?

Adding this support should be very straightforward.

[bcmills]

(CC @golang/windows)

[bcmills]

SetFilePointerEx seems like an obvious addition: we already have SetFilePointer and AFAICT the only difference is the operand size.

I also see no reason not to add SetFileValidData. Want to send a CL?

[kanak-clumio]

Sure! I've not contributed to golang yet and am used to github and not gerrit. Let me look into setting up the repo and will create a PR with this change

[gopherbot]

Change https://go.dev/cl/517575 mentions this issue: Windows: Add SetFileValidData and SetFilePointerEx