all: move network monitoring from wgengine/monitor to net/netmon

We're using it in more and more places, and it's not really specific to
our use of Wireguard (and does more just link/interface monitoring).

Also removes the separate interface we had for it in sockstats -- it's
a small enough package (we already pull in all of its dependencies
via other paths) that it's not worth the extra complexity.

Updates #7621
Updates #7850

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/interfaces/interfaces.go

@@ -351,12 +351,6 @@ func (s *State) String() string {
 	return sb.String()
 }
 
-// ChangeFunc is a callback function (usually registered with
-// wgengine/monitor's Mon) that's called when the network
-// changed. The changed parameter is whether the network changed
-// enough for State to have changed since the last callback.
-type ChangeFunc func(changed bool, state *State)
-
 // An InterfaceFilter indicates whether EqualFiltered should use i when deciding whether two States are equal.
 // ips are all the IPPrefixes associated with i.
 type InterfaceFilter func(i Interface, ips []netip.Prefix) bool

net/interfaces/interfaces_linux.go

@@ -167,8 +167,8 @@ func defaultRoute() (d DefaultRouteDetails, err error) {
 	// /proc/net/route. Use netlink to find the default route.
 	//
 	// TODO(bradfitz): this allocates a fair bit. We should track
-	// this in wgengine/monitor instead and have
-	// interfaces.GetState take a link monitor or similar so the
+	// this in net/interfaces/monitor instead and have
+	// interfaces.GetState take a netmon.Monitor or similar so the
 	// routing table can be cached and the monitor's existing
 	// subscription to route changes can update the cached state,
 	// rather than querying the whole thing every time like