ipn, ipnserver: only require sudo on Linux for mutable CLI actions

This partially reverts d6e9fb1df0, which modified the permissions
on the tailscaled Unix socket and thus required "sudo tailscale" even
for "tailscale status".

Instead, open the permissions back up (on Linux only) but have the
server look at the peer creds and only permit read-only actions unless
you're root.

In the future we'll also have a group that can do mutable actions.

On OpenBSD and FreeBSD, the permissions on the socket remain locked
down to 0600 from d6e9fb1df0.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/message_test.go

@@ -6,6 +6,7 @@ package ipn
 
 import (
 	"bytes"
+	"context"
 	"testing"
 	"time"
 
@@ -81,7 +82,7 @@ func TestClientServer(t *testing.T) {
 		serverToClientCh <- append([]byte{}, b...)
 	}
 	clientToServer := func(b []byte) {
-		bs.GotCommandMsg(b)
+		bs.GotCommandMsg(context.TODO(), b)
 	}
 	slogf := func(fmt string, args ...interface{}) {
 		t.Logf("s: "+fmt, args...)