ipn/ipnstate: add TailnetName to status

RELNOTE=tailscale status --json | jq .TailnetName

Fixes tailscale/corp#3665

Change-Id: I85de027ba2781eb31ee1e0c5ab913b0dfa5b4c86
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -381,6 +381,7 @@ func (b *LocalBackend) updateStatus(sb *ipnstate.StatusBuilder, extraLocked func
 		if b.netMap != nil {
 			s.MagicDNSSuffix = b.netMap.MagicDNSSuffix()
 			s.CertDomains = append([]string(nil), b.netMap.DNS.CertDomains...)
+			s.TailnetName = b.netMap.Domain
 		}
 	})
 	sb.MutateSelfStatus(func(ss *ipnstate.PeerStatus) {

ipn/ipnstate/ipnstate.go

@@ -33,6 +33,10 @@ type Status struct {
 	//  "Starting", "Running".
 	BackendState string
 
+	// TailnetName is the name of the network that's currently in
+	// use.
+	TailnetName string
+
 	AuthURL      string       // current URL provided by control to authorize client
 	TailscaleIPs []netaddr.IP // Tailscale IP(s) assigned to this node
 	Self         *PeerStatus