Automatic merge from submit-queue
go-client: Use httpcache client for all requests, even though only openapi returns ETags for caching
**What this PR does / why we need it**: Use HTTP ETag for caching Swagger spec download
This also adds a new command-line flag "cachedir" to specify where the cache should keep its file. It defaults to `$HOME/.kube/http-cache`.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: partly #38637
**Special notes for your reviewer**:
Because this adds a bunch of dependencies, and removes a couple of files, I do recommend reading each commit individually.
**Release note**:
```release-note
```
Kubernetes-commit: fc89743dca6b563063b74728c3b28100cf674d9d
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
```
Kubernetes-commit: 5d24a2c19923d6da46110b827619f4b21cf689ac
Kubernetes Publisher