ipn/ipnserver: add Server.LocalBackend accessor

Was done as part of e6fbc0cd54 for ssh
work, but wasn't committed yet. Including it here both to minimize the
ssh diff size, and because I need it for a separate change.

Change-Id: If6eb54a2ca7150ace96488ed14582c2c05ca3422
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server.go

@@ -107,6 +107,9 @@ type Server struct {
 	disconnectSub  map[chan<- struct{}]struct{} // keys are subscribers of disconnects
 }
 
+// LocalBackend returns the server's LocalBackend.
+func (s *Server) LocalBackend() *ipnlocal.LocalBackend { return s.b }
+
 // connIdentity represents the owner of a localhost TCP or unix socket connection.
 type connIdentity struct {
 	Conn       net.Conn