tailcfg: update node display name fields and methods (#1207)

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

Consolidates the node display name logic from each of the clients into
tailcfg.Node. UI clients can use these names directly, rather than computing
them independently.

control/controlclient/netmap.go

@@ -24,13 +24,12 @@ import (
 type NetworkMap struct {
 	// Core networking
 
+	SelfNode   *tailcfg.Node
 	NodeKey    tailcfg.NodeKey
 	PrivateKey wgkey.Private
 	Expiry     time.Time
 	// Name is the DNS name assigned to this node.
-	Name string
-	// DisplayName is the title to show for the node in client UIs.
-	DisplayName   string
+	Name          string
 	Addresses     []netaddr.IPPrefix
 	LocalPort     uint16 // used for debugging
 	MachineStatus tailcfg.MachineStatus