libnet: Make sure network names are unique
**- What I did**

Fixes #18864, #20648, #33561, #40901 (and probably more).

[This GH comment][1] makes clear network name uniqueness has never been enforced due to the eventually consistent nature of Classic Swarm datastores:

> there is no guaranteed way to check for duplicates across a cluster of docker hosts.

And this is further confirmed by other comments made by @mrjana in that same issue, eg. [this one][2]:

> we want to adopt a schema which can pave the way in the future for a completely decentralized cluster of docker hosts (if scalability is needed).

This decentralized model is what Classic Swarm was trying to be. It's been superseded since then by Docker Swarm, which has a centralized control plane.

To circumvent this drawback, the `NetworkCreate` endpoint accepts a `CheckDuplicate` flag. However it's not perfectly reliable as it won't catch concurrent requests.

Due to this design decision, API clients like Compose have to implement workarounds to make sure names are really unique (eg. docker/compose#9585). And the daemon itself has seen a string of issues due to that decision, including some that aren't fixed to this day (for instance moby/moby#40901):

> The problem is, that if you specify a network for a container using the ID, it will add that network to the container but it will then change it to reference the network by using the name.

To summarize, this "feature" is broken, has no practical use and is a source of pain for Docker users and API consumers. So let's just remove it for _all_ API versions.

[1]: https://github.com/moby/moby/issues/18864#issuecomment-167201414
[2]: https://github.com/moby/moby/issues/18864#issuecomment-167202589

**- A picture of a cute animal (not mandatory but encouraged)**

<img src="https://i.natgeofe.com/k/80385d2b-cf77-43eb-9589-a2c9229401bd/quokka-selfie-smile.jpg" width=50% height=50% />
client/network_create_test.go
package client // import "github.com/docker/docker/client"

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"strings"
	"testing"

	"github.com/docker/docker/api/types"
	"github.com/docker/docker/errdefs"
	"gotest.tools/v3/assert"
	is "gotest.tools/v3/assert/cmp"
)

func TestNetworkCreateError(t *testing.T) {
	client := &Client{
		client: newMockClient(errorMock(http.StatusInternalServerError, "Server error")),
	}

	_, err := client.NetworkCreate(context.Background(), "mynetwork", types.NetworkCreate{})
	assert.Check(t, is.ErrorType(err, errdefs.IsSystem))
}

func TestNetworkCreate(t *testing.T) {
	expectedURL := "/networks/create"

	client := &Client{
		client: newMockClient(func(req *http.Request) (*http.Response, error) {
			if !strings.HasPrefix(req.URL.Path, expectedURL) {
				return nil, fmt.Errorf("Expected URL '%s', got '%s'", expectedURL, req.URL)
			}

			if req.Method != http.MethodPost {
				return nil, fmt.Errorf("expected POST method, got %s", req.Method)
			}

			content, err := json.Marshal(types.NetworkCreateResponse{
				ID:      "network_id",
				Warning: "warning",
			})
			if err != nil {
				return nil, err
			}
			return &http.Response{
				StatusCode: http.StatusOK,
				Body:       io.NopCloser(bytes.NewReader(content)),
			}, nil
		}),
	}

	networkResponse, err := client.NetworkCreate(context.Background(), "mynetwork", types.NetworkCreate{
		CheckDuplicate: true,
		Driver:         "mydriver",
		EnableIPv6:     true,
		Internal:       true,
		Options: map[string]string{
			"opt-key": "opt-value",
		},
	})
	if err != nil {
		t.Fatal(err)
	}
	if networkResponse.ID != "network_id" {
		t.Fatalf("expected networkResponse.ID to be 'network_id', got %s", networkResponse.ID)
	}
	if networkResponse.Warning != "warning" {
		t.Fatalf("expected networkResponse.Warning to be 'warning', got %s", networkResponse.Warning)
	}
}

package client // import "github.com/docker/docker/client"

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"strings"
	"testing"

	"github.com/docker/docker/api/types"
	"github.com/docker/docker/errdefs"
	"gotest.tools/v3/assert"
	is "gotest.tools/v3/assert/cmp"
)

func TestNetworkCreateError(t *testing.T) {
	client := &Client{
		client: newMockClient(errorMock(http.StatusInternalServerError, "Server error")),
	}

	_, err := client.NetworkCreate(context.Background(), "mynetwork", types.NetworkCreate{})
	assert.Check(t, is.ErrorType(err, errdefs.IsSystem))
}

func TestNetworkCreate(t *testing.T) {
	expectedURL := "/networks/create"

	client := &Client{
		client: newMockClient(func(req *http.Request) (*http.Response, error) {
			if !strings.HasPrefix(req.URL.Path, expectedURL) {
				return nil, fmt.Errorf("Expected URL '%s', got '%s'", expectedURL, req.URL)
			}

			if req.Method != http.MethodPost {
				return nil, fmt.Errorf("expected POST method, got %s", req.Method)
			}

			content, err := json.Marshal(types.NetworkCreateResponse{
				ID:      "network_id",
				Warning: "warning",
			})
			if err != nil {
				return nil, err
			}
			return &http.Response{
				StatusCode: http.StatusOK,
				Body:       io.NopCloser(bytes.NewReader(content)),
			}, nil
		}),
	}

	networkResponse, err := client.NetworkCreate(context.Background(), "mynetwork", types.NetworkCreate{
		Driver:     "mydriver",
		EnableIPv6: true,
		Internal:   true,
		Options: map[string]string{
			"opt-key": "opt-value",
		},
	})
	if err != nil {
		t.Fatal(err)
	}
	if networkResponse.ID != "network_id" {
		t.Fatalf("expected networkResponse.ID to be 'network_id', got %s", networkResponse.ID)
	}
	if networkResponse.Warning != "warning" {
		t.Fatalf("expected networkResponse.Warning to be 'warning', got %s", networkResponse.Warning)
	}
}

We need to update `client.NetworkCreate` to always send `CheckDuplicate: true` when creating an API request to an older API version.

Otherwise, older API's without this patch will use `CheckDuplicate: false` (on the daemon side), which would allow for duplicate networks to be created without error.