ipn/ipnstate, cmd/tailscale: add Online bool to tailscale status & --json

Fixes #3533

Change-Id: I2f6f0d712cf3f987fba1c15be74cdb5c8d565f04
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnstate/ipnstate.go

@@ -100,8 +100,9 @@ type PeerStatus struct {
 	TxBytes        int64
 	Created        time.Time // time registered with tailcontrol
 	LastWrite      time.Time // time last packet sent
-	LastSeen       time.Time // last seen to tailcontrol
+	LastSeen       time.Time // last seen to tailcontrol; only present if offline
 	LastHandshake  time.Time // with local wireguard
+	Online         bool      // whether node is connected to the control plane
 	KeepAlive      bool
 	ExitNode       bool // true if this is the currently selected exit node.
 	ExitNodeOption bool // true if this node can be an exit node (offered && approved)
@@ -276,6 +277,9 @@ func (sb *StatusBuilder) AddPeer(peer key.NodePublic, st *PeerStatus) {
 	if v := st.LastWrite; !v.IsZero() {
 		e.LastWrite = v
 	}
+	if st.Online {
+		e.Online = true
+	}
 	if st.InNetworkMap {
 		e.InNetworkMap = true
 	}