wgengine: incremental update of peers on network map change

This is the first, and easier, part of incremental wireguard-go
reconfiguration. It means that a new node appearing on the
network does not cause all existing nodes to re-handshake with
the other nodes they are talking to.

(This code has been running on hello.ipn.dev for a few weeks and
peers have successfully reconnected to it through many network
map updates.)

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

wgengine/userspace.go

@@ -231,9 +231,8 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, dnsDomains []string) error
 	}
 	e.lastReconfig = rc
 
-	r := bufio.NewReader(strings.NewReader(uapi))
-	if err = e.wgdev.IpcSetOperation(r); err != nil {
-		e.logf("IpcSetOperation: %v\n", err)
+	if err := e.wgdev.Reconfig(cfg); err != nil {
+		e.logf("wgdev.Reconfig: %v\n", err)
 		return err
 	}
 
@@ -458,7 +457,8 @@ func (e *userspaceEngine) RequestStatus() {
 }
 
 func (e *userspaceEngine) Close() {
-	e.Reconfig(&wgcfg.Config{}, nil)
+	r := bufio.NewReader(strings.NewReader(""))
+	e.wgdev.IpcSetOperation(r)
 	e.linkMon.Close()
 	e.router.Close()
 	e.magicConn.Close()