all: update to Go 1.18

Bump go.mod and README.

Switch to upstream net/netip.

Use strings.Cut.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/allowedips.go

@@ -11,10 +11,9 @@ import (
 	"errors"
 	"math/bits"
 	"net"
+	"net/netip"
 	"sync"
 	"unsafe"
-
-	"golang.zx2c4.com/go118/netip"
 )
 
 type parentIndirection struct {

device/allowedips_rand_test.go

@@ -8,10 +8,9 @@ package device
 import (
 	"math/rand"
 	"net"
+	"net/netip"
 	"sort"
 	"testing"
-
-	"golang.zx2c4.com/go118/netip"
 )
 
 const (

device/allowedips_test.go

@@ -8,9 +8,8 @@ package device
 import (
 	"math/rand"
 	"net"
+	"net/netip"
 	"testing"
-
-	"golang.zx2c4.com/go118/netip"
 )
 
 type testPairCommonBits struct {

device/device_test.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"math/rand"
+	"net/netip"
 	"runtime"
 	"runtime/pprof"
 	"sync"
@@ -18,7 +19,6 @@ import (
 	"testing"
 	"time"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/conn"
 	"golang.zx2c4.com/wireguard/conn/bindtest"
 	"golang.zx2c4.com/wireguard/tun/tuntest"

device/endpoint_test.go

@@ -7,8 +7,7 @@ package device
 
 import (
 	"math/rand"
-
-	"golang.zx2c4.com/go118/netip"
+	"net/netip"
 )
 
 type DummyEndpoint struct {

device/uapi.go

@@ -12,13 +12,13 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/netip"
 	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/ipc"
 )
 
@@ -161,12 +161,10 @@ func (device *Device) IpcSetOperation(r io.Reader) (err error) {
 			peer.handlePostConfig()
 			return nil
 		}
-		parts := strings.Split(line, "=")
-		if len(parts) != 2 {
-			return ipcErrorf(ipc.IpcErrorProtocol, "failed to parse line %q, found %d =-separated parts, want 2", line, len(parts))
+		key, value, ok := strings.Cut(line, "=")
+		if !ok {
+			return ipcErrorf(ipc.IpcErrorProtocol, "failed to parse line %q", line)
 		}
-		key := parts[0]
-		value := parts[1]
 
 		if key == "public_key" {
 			if deviceConfig {