User of `nsutil` library should be able to do the following and for it
to work:
```
var errno syscall.Errno
if errors.As(err, &errno) {
if errno == unix.EBUSY { ... }
}
```
This commit fixes that issue.
We bring in `containernetworking/plugins` for the contents of a single file,
which we use in a few places for running a goroutine in a specific network
namespace. This code hasn't needed an update in a couple of years, and a good
chunk of what we need was previously vendored into `client/lib/nsutil`
already.
Updating the library via dependabot is causing errors in Docker driver tests
because it updates a lot of transient dependencies, and it's bringing in a pile
of new transient dependencies like opentelemetry. Avoid this problem going
forward by vendoring the remaining code we hadn't already.
Ref: https://github.com/hashicorp/nomad/pull/20146
