p2p/dnsdisc: add implementation of EIP-1459 (#20094)

This adds an implementation of node discovery via DNS TXT records to the
go-ethereum library. The implementation doesn't match EIP-1459 exactly,
the main difference being that this implementation uses separate merkle
trees for tree links and ENRs. The EIP will be updated to match p2p/dnsdisc.

To maintain DNS trees, cmd/devp2p provides a frontend for the p2p/dnsdisc
library. The new 'dns' subcommands can be used to create, sign and deploy DNS
discovery trees.

cmd/devp2p/main.go

@@ -20,8 +20,10 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sort"
 
 	"github.com/ethereum/go-ethereum/internal/debug"
+	"github.com/ethereum/go-ethereum/p2p/enode"
 	"github.com/ethereum/go-ethereum/params"
 	"gopkg.in/urfave/cli.v1"
 )
@@ -57,12 +59,38 @@ func init() {
 	app.Commands = []cli.Command{
 		enrdumpCommand,
 		discv4Command,
+		dnsCommand,
 	}
 }
 
 func main() {
-	if err := app.Run(os.Args); err != nil {
-		fmt.Fprintln(os.Stderr, err)
-		os.Exit(1)
-	}
+	exit(app.Run(os.Args))
+}
+
+// commandHasFlag returns true if the current command supports the given flag.
+func commandHasFlag(ctx *cli.Context, flag cli.Flag) bool {
+	flags := ctx.FlagNames()
+	sort.Strings(flags)
+	i := sort.SearchStrings(flags, flag.GetName())
+	return i != len(flags) && flags[i] == flag.GetName()
+}
+
+// getNodeArg handles the common case of a single node descriptor argument.
+func getNodeArg(ctx *cli.Context) *enode.Node {
+	if ctx.NArg() != 1 {
+		exit("missing node as command-line argument")
+	}
+	n, err := parseNode(ctx.Args()[0])
+	if err != nil {
+		exit(err)
+	}
+	return n
+}
+
+func exit(err interface{}) {
+	if err == nil {
+		os.Exit(0)
+	}
+	fmt.Fprintln(os.Stderr, err)
+	os.Exit(1)
 }