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/dns/resolver/macios_ext.go

@@ -9,15 +9,15 @@ import (
 	"errors"
 	"net"
 
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/netns"
-	"tailscale.com/wgengine/monitor"
 )
 
 func init() {
 	initListenConfig = initListenConfigNetworkExtension
 }
 
-func initListenConfigNetworkExtension(nc *net.ListenConfig, mon *monitor.Mon, tunName string) error {
+func initListenConfigNetworkExtension(nc *net.ListenConfig, mon *netmon.Monitor, tunName string) error {
 	nif, ok := mon.InterfaceState().Interface[tunName]
 	if !ok {
 		return errors.New("utun not found")