genkeys, yggdrasilctl: Use pledge(2) on OpenBSD (#1193)

Restrict system operations of CLI tools with
https://man.openbsd.org/pledge.2.

https://pkg.go.dev/suah.dev/protect abstracts the OS specific code, i.e.
is a NOOP on non-OpenBSD systems.

This PR is to gauge upstream interest in this direction; my OpenBSD port
of yggdrasil already pledges the daemon,
resulting in minimal runtime privileges, but there are still a few rough
edges:

https://github.com/jasperla/openbsd-wip/blob/master/net/yggdrasil/patches/patch-cmd_yggdrasil_main_go#L80

---------

Co-authored-by: Neil <git@neilalexander.dev>

cmd/genkeys/main.go

@@ -18,6 +18,8 @@ import (
 	"runtime"
 	"time"
 
+	"suah.dev/protect"
+
 	"github.com/yggdrasil-network/yggdrasil-go/src/address"
 )
 
@@ -27,6 +29,10 @@ type keySet struct {
 }
 
 func main() {
+	if err := protect.Pledge("stdio"); err != nil {
+		panic(err)
+	}
+
 	threads := runtime.GOMAXPROCS(0)
 	fmt.Println("Threads:", threads)
 	start := time.Now()

cmd/yggdrasilctl/main.go

@@ -13,6 +13,8 @@ import (
 	"strings"
 	"time"
 
+	"suah.dev/protect"
+
 	"github.com/olekukonko/tablewriter"
 	"github.com/yggdrasil-network/yggdrasil-go/src/admin"
 	"github.com/yggdrasil-network/yggdrasil-go/src/core"
@@ -22,6 +24,11 @@ import (
 )
 
 func main() {
+	// read config, speak DNS/TCP and/or over a UNIX socket
+	if err := protect.Pledge("stdio rpath inet unix dns"); err != nil {
+		panic(err)
+	}
+
 	// makes sure we can use defer and still return an error code to the OS
 	os.Exit(run())
 }
@@ -78,6 +85,11 @@ func run() int {
 		panic(err)
 	}
 
+	// config and socket are done, work without unprivileges
+	if err := protect.Pledge("stdio"); err != nil {
+		panic(err)
+	}
+
 	logger.Println("Connected")
 	defer conn.Close()