proto/msgpack: Fix handshake magic

This fixes the basic NEO handshake in msgpack encoding.

Without this fix NEO/go client test fails with a timeout. In NEO/go log
we can see

```
Log file created at: 2024/08/20 12:50:11
Running on machine: ubuntu
Binary: Built with gc go1.18.1 for linux/amd64
Log line format: [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line] msg
I0820 12:50:11.300574 1137321 client.go:418] neo: open neo://1@127.0.0.1:23004: [
I0820 12:50:11.301130 1137321 mastered.go:119] C?: talk master(127.0.0.1:23004): [
I0820 12:50:11.301140 1137321 connect.go:115] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): [
I0820 12:50:11.303723 1137321 connect.go:119] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): ] (127.0.0.1:42388 - 127.0.0.1:23004: handshake (client): rx hello reply: unexpected EOF)
W0820 12:50:11.303729 1137321 mastered.go:127] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): 127.0.0.1:42388 - 127.0.0.1:23004: handshake (client): rx hello reply: unexpected EOF
I0820 12:50:12.304076 1137321 connect.go:115] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): [
I0820 12:50:12.306096 1137321 connect.go:119] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): ] (127.0.0.1:42410 - 127.0.0.1:23004: handshake (client): rx hello reply: unexpected EOF)
W0820 12:50:12.306102 1137321 mastered.go:127] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): 127.0.0.1:42410 - 127.0.0.1:23004: handshake (client): rx hello reply: unexpected EOF
I0820 12:50:13.306311 1137321 connect.go:115] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): [
I0820 12:50:13.307818 1137321 connect.go:119] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): ] (127.0.0.1:56834 - 127.0.0.1:23004: handshake (client): rx hello reply: unexpected EOF)
W0820 12:50:13.307824 1137321 mastered.go:127] C?: talk master(127.0.0.1:23004): dial 127.0.0.1:23004 (MASTER): 127.0.0.1:56834 - 127.0.0.1:23004: handshake (client): rx hello reply: unexpected EOF
```

And in NEO/py logs we can see:

```
2024-08-20 12:51:35.5152 DEBUG     M1         Rejecting non-NEO <ServerConnection(nid=None, address=127.0.0.1:35404, handler=IdentificationHandler, fd=18, server) at 7f9417467510>
2024-08-20 12:51:35.5153 DEBUG     M1         connection closed for <ServerConnection(nid=None, address=127.0.0.1:35404, handler=IdentificationHandler, closed, server) at 7f9417467510>
2024-08-20 12:51:36.5161 DEBUG     M1         accepted a connection from 127.0.0.1:35408
2024-08-20 12:51:36.5168 DEBUG     M1         connection completed for <ServerConnection(nid=None, address=127.0.0.1:35408, handler=IdentificationHandler, fd=18, server) at 7f9417467210> (from 127.0.0
```

NEO/py defines its handshake packet here:

https://lab.nexedi.com/nexedi/neoppod/-/blob/9d0bf97a1/neo/lib/protocol.py#L27

It's encoded form can be simply revealed with the help of a python shell:

```python
>>> from neo.lib import protocol
>>> protocol.HANDSHAKE_PACKET
'\x92\xa3NEO\x00'
```

go/neo/neonet/newlink.go

@@ -216,7 +216,7 @@ func _handshakeServer(ctx context.Context, conn net.Conn, version uint32) (enc p
 // handshake hello:
 //
 // - 00 00 00 <ver>	for 'N' encoding, and
-// - 92 c4 03 NEO ...	for 'M' encoding (= msgpack of (b"NEO", <ver>))
+// - 92 a3 NEO ...	for 'M' encoding (= msgpack of ("NEO", <ver>))
 //
 // the first byte is different from TLS handshake (0x16).
 
@@ -234,9 +234,9 @@ func txHello(errctx string, conn net.Conn, version uint32, enc proto.Encoding) (
 		b[3] = uint8(version)
 
 	case 'M':
-		// (b"NEO", <V>) encoded as msgpack (= 92 c4 03 NEO int(<V>))
+		// ("NEO", <V>) encoded as msgpack (= 92 a3 NEO int(<V>))
 		b = msgp.AppendArrayHeader(b, 2)       // 92
-		b = msgp.AppendBytes(b, []byte("NEO")) // c4 03 NEO
+		b = msgp.AppendString(b, "NEO")        // a3 NEO
 		b = msgp.AppendUint32(b, version)      // u?intX version
 
 	default:
@@ -269,14 +269,14 @@ func rxHello(errctx string, rx *xbufReader) (enc proto.Encoding, version uint32,
 		peerEnc = 'N'
 		peerVer = uint32(b[3])
 
-	case bytes.Equal(b, []byte{0x92, 0xc4, 3, 'N'}): // start of "fixarray<2> bin8 'N | EO' ...
-		b = append(b, []byte{0,0}...)
+	case bytes.Equal(b, []byte{0x92, 0xa3, 'N', 'E'}): // start of "fixarray<2> "fixstr3 'NE | O' ...
+		b = append(b, []byte{0}...)
 		_, err = io.ReadFull(rx, b[4:])
 		err = xio.NoEOF(err)
 		if err != nil {
 			return 0, 0, err
 		}
-		if !bytes.Equal(b[4:], []byte{'E','O'}) {
+		if !bytes.Equal(b[4:5], []byte{'O'}) {
 			badMagic = true
 			break
 		}
@@ -287,7 +287,6 @@ func rxHello(errctx string, rx *xbufReader) (enc proto.Encoding, version uint32,
 		if err != nil {
 			return 0, 0, fmt.Errorf("M: recv peer version: %s", err) // XXX + "read magic" ctx
 		}
-
 	default:
 		badMagic = true
 	}