add state to socketLink to prevent errors

[brycekahle]

What does this PR do?

Adds attached state to socketLink

Motivation

If you called Pause() then Close(), it would result in an error

Additional Notes

Describe how to test your changes

[mbertrone]

suggestion (non-blocking): the ENOENT branch reconciles state to attached=false but still returns the error, so an already-detached socket surfaces a spurious error through Close()/Pause() into the joined result at probe.go:703. Not a regression, and only reachable when kernel and tracked state diverge, but returning the error here works against the idempotent-detach intent of this PR. If that divergence is the expected "already detached" case, return nil after clearing the flag:

if errors.Is(err, unix.ENOENT) {
	s.attached = false
	return nil
}
return err

[brycekahle]

I think surfacing the error when kernel and tracked state diverge is important