main, lib: Build in standalone etcd

Standalone etcd is useful for when we don't want to use the embedded
version to make it easier to deploy somewhere or for testing.

This pulls in about the same amount of code since we already embedded
etcd previously. Since the embedded etcd feature of mgmt is not very
stable, we'll add this for now.

main.go

@@ -23,6 +23,7 @@ import (
 	"os"
 
 	mgmt "github.com/purpleidea/mgmt/lib"
+	"go.etcd.io/etcd/server/v3/etcdmain"
 )
 
 // These constants are some global variables that are used throughout the code.
@@ -41,6 +42,19 @@ var (
 var copying string
 
 func main() {
+	// embed etcd completely
+	if len(os.Args) >= 2 && os.Args[1] == "etcd" {
+		args := []string{}
+		for i, s := range os.Args {
+			if i == 0 { // pop off our argv[0] and let `etcd` be it
+				continue
+			}
+			args = append(args, s)
+		}
+		etcdmain.Main(args) // this will os.Exit
+		return              // for safety
+	}
+
 	flags := mgmt.Flags{
 		Debug:   Debug,
 		Verbose: Verbose,