[libc][NFC] clean internal fd handling (#143991)
The previous internal fcntl implementation modified errno directly, this
patch fixes that. This patch also moves open and close into OSUtil since
they are used in multiple places. There are more places that need
similar cleanup but only got comments in this patch to keep it
relatively reviewable.
Related to: https://github.com/llvm/llvm-project/issues/143937
