Create a proper Lock/Unlock

[rwxrob]

Just found out that there is a proposal to externalize good file
locking: golang/go#33974, but it hasn't seen
attention in a while. I think "good" locking will have to wait. For now,
I'm going to just use what kubeconfig uses (which is far from good).

So many seriously bad implementations of file locking exist. None of
them take into account the potential race condition of just looking for
the existence of a file (this ain't Windows). The proper way to do it is
to flock the file in addition to checking if it exists.

Here's an example of a failed lock
design that is all
throughout the Kubernetes code base (and Kind decided to copy it). Any
seasoned UNIX systems developer will immediately see the failures here:

package kubeconfig

import (
	"os"
	"path/filepath"
)

// these are from
// https://github.com/kubernetes/client-go/blob/611184f7c43ae2d520727f01d49620c7ed33412d/tools/clientcmd/loader.go#L439-L440

func lockFile(filename string) error {
	// Make sure the dir exists before we try to create a lock file.
	dir := filepath.Dir(filename)
	if _, err := os.Stat(dir); os.IsNotExist(err) {
		if err = os.MkdirAll(dir, 0755); err != nil {
			return err
		}
	}
	f, err := os.OpenFile(lockName(filename), os.O_CREATE|os.O_EXCL, 0)
	if err != nil {
		return err
	}
	f.Close()
	return nil
}

func unlockFile(filename string) error {
	return os.Remove(lockName(filename))
}

func lockName(filename string) string {
	return filename + ".lock"
}

I need to add a good Lock/Unlock to this package just so people can
see the difference for themselves and compare.