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>

tun/netstack/examples/http_client.go

@@ -12,8 +12,8 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"net/netip"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/conn"
 	"golang.zx2c4.com/wireguard/device"
 	"golang.zx2c4.com/wireguard/tun/netstack"

tun/netstack/examples/http_server.go

@@ -13,8 +13,8 @@ import (
 	"log"
 	"net"
 	"net/http"
+	"net/netip"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/conn"
 	"golang.zx2c4.com/wireguard/device"
 	"golang.zx2c4.com/wireguard/tun/netstack"

tun/netstack/examples/ping_client.go

@@ -12,12 +12,12 @@ import (
 	"bytes"
 	"log"
 	"math/rand"
+	"net/netip"
 	"time"
 
 	"golang.org/x/net/icmp"
 	"golang.org/x/net/ipv4"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/conn"
 	"golang.zx2c4.com/wireguard/device"
 	"golang.zx2c4.com/wireguard/tun/netstack"

tun/netstack/tun.go

@@ -13,13 +13,13 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/netip"
 	"os"
 	"regexp"
 	"strconv"
 	"strings"
 	"time"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/tun"
 
 	"golang.org/x/net/dns/dnsmessage"

tun/tuntest/tuntest.go

@@ -8,9 +8,9 @@ package tuntest
 import (
 	"encoding/binary"
 	"io"
+	"net/netip"
 	"os"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/tun"
 )