Automatic merge from submit-queue

Switch from package syscall to golang.org/x/sys/unix

**What this PR does / why we need it**:

The syscall package is locked down and the comment in https://github.com/golang/go/blob/master/src/syscall/syscall.go#L21-L24 advises to switch code to use the corresponding package from golang.org/x/sys. This PR does so and replaces usage of package syscall with package golang.org/x/sys/unix where applicable. This will also allow to get updates and fixes
without having to use a new go version.

In order to get the latest functionality, golang.org/x/sys/ is re-vendored. This also allows to use Eventfd() from this package instead of calling the eventfd() C function.

**Special notes for your reviewer**:

This follows previous works in other Go projects, see e.g. moby/moby#33399, cilium/cilium#588

**Release note**:

```release-note
NONE
```