wgengine{,/monitor}: move interface state fetching/comparing to monitor

Gets it out of wgengine so the Engine isn't responsible for being a
callback registration hub for it.

This also removes the Engine.LinkChange method, as it's no longer
necessary.  The monitor tells us about changes; it doesn't seem to
need any help. (Currently it was only used by Swift, but as of
14dc790137 we just do the same from Go)

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

wgengine/wgengine.go

@@ -9,10 +9,10 @@ import (
 
 	"inet.af/netaddr"
 	"tailscale.com/ipn/ipnstate"
-	"tailscale.com/net/interfaces"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine/filter"
+	"tailscale.com/wgengine/monitor"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/tsdns"
 	"tailscale.com/wgengine/wgcfg"
@@ -72,6 +72,9 @@ type Engine interface {
 	// WireGuard status changes.
 	SetStatusCallback(StatusCallback)
 
+	// GetLinkMonitor returns the link monitor.
+	GetLinkMonitor() *monitor.Mon
+
 	// RequestStatus requests a WireGuard status update right
 	// away, sent to the callback registered via SetStatusCallback.
 	RequestStatus()
@@ -86,18 +89,6 @@ type Engine interface {
 	// TODO: return an error?
 	Wait()
 
-	// LinkChange informs the engine that the system network
-	// link has changed. The isExpensive parameter is set on links
-	// where sending packets uses substantial power or money,
-	// such as mobile data on a phone.
-	//
-	// LinkChange should be called whenever something changed with
-	// the network, no matter how minor. The implementation should
-	// look at the state of the network and decide whether the
-	// change from before is interesting enough to warrant taking
-	// action on.
-	LinkChange(isExpensive bool)
-
 	// SetDERPMap controls which (if any) DERP servers are used.
 	// If nil, DERP is disabled. It starts disabled until a DERP map
 	// is configured.
@@ -120,13 +111,6 @@ type Engine interface {
 	// new NetInfo summary is available.
 	SetNetInfoCallback(NetInfoCallback)
 
-	// SetLinkChangeCallback sets the function to call when the
-	// link state changes.
-	// The provided function is run in a new goroutine once upon
-	// initial call (if the engine has a known link state) and
-	// upon any change.
-	SetLinkChangeCallback(func(major bool, newState *interfaces.State))
-
 	// DiscoPublicKey gets the public key used for path discovery
 	// messages.
 	DiscoPublicKey() tailcfg.DiscoKey