tsd: add package with System type to unify subsystem init, discovery

This is part of an effort to clean up tailscaled initialization between
tailscaled, tailscaled Windows service, tsnet, and the mac GUI.

Updates #8036

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

ipn/ipnserver/server.go

@@ -37,7 +37,7 @@ import (
 type Server struct {
 	lb           atomic.Pointer[ipnlocal.LocalBackend]
 	logf         logger.Logf
-	netMon       *netmon.Monitor // optional; nil means interfaces will be looked up on-demand
+	netMon       *netmon.Monitor // must be non-nil
 	backendLogID logid.PublicID
 	// resetOnZero is whether to call bs.Reset on transition from
 	// 1->0 active HTTP requests. That is, this is whether the backend is
@@ -410,14 +410,15 @@ func (s *Server) addActiveHTTPRequest(req *http.Request, ci *ipnauth.ConnIdentit
 }
 
 // New returns a new Server.
-// The netMon parameter is optional; if non-nil it's used to do faster interface
-// lookups.
 //
 // To start it, use the Server.Run method.
 //
 // At some point, either before or after Run, the Server's SetLocalBackend
 // method must also be called before Server can do anything useful.
 func New(logf logger.Logf, logID logid.PublicID, netMon *netmon.Monitor) *Server {
+	if netMon == nil {
+		panic("nil netMon")
+	}
 	return &Server{
 		backendLogID: logID,
 		logf:         logf,