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>

wgengine/wgengine.go

@@ -10,7 +10,6 @@ import (
 
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
-	"tailscale.com/net/netmon"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
@@ -92,9 +91,6 @@ type Engine interface {
 	// WireGuard status changes.
 	SetStatusCallback(StatusCallback)
 
-	// GetNetMon returns the network monitor.
-	GetNetMon() *netmon.Monitor
-
 	// RequestStatus requests a WireGuard status update right
 	// away, sent to the callback registered via SetStatusCallback.
 	RequestStatus()