uapi: insert peer version placeholder

While we don't want people to ever use old protocols, people will
complain if the API "changes", so explicitly make the unset protocol
mean the latest, and add a dummy mechanism of specifying the protocol on
a per-peer basis, which we hope nobody actually ever uses.

uapi.go

@@ -75,6 +75,7 @@ func ipcGetOperation(device *Device, socket *bufio.ReadWriter) *IPCError {
 
 			send("public_key=" + peer.handshake.remoteStatic.ToHex())
 			send("preshared_key=" + peer.handshake.presharedKey.ToHex())
+			send("protocol_version=1")
 			if peer.endpoint != nil {
 				send("endpoint=" + peer.endpoint.DstToString())
 			}
@@ -362,6 +363,13 @@ func ipcSetOperation(device *Device, socket *bufio.ReadWriter) *IPCError {
 				ones, _ := network.Mask.Size()
 				device.allowedips.Insert(network.IP, uint(ones), peer)
 
+			case "protocol_version":
+
+				if value != "1" {
+					logError.Println("Invalid protocol version:", value)
+					return &IPCError{Code: ipcErrorInvalid}
+				}
+
 			default:
 				logError.Println("Invalid UAPI peer key:", key)
 				return &IPCError{Code: ipcErrorInvalid}