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/sockstats/sockstats_noop.go

@@ -8,6 +8,7 @@ package sockstats
 import (
 	"context"
 
+	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 )
 
@@ -29,7 +30,7 @@ func getValidation() *ValidationSockStats {
 	return nil
 }
 
-func setLinkMonitor(lm LinkMonitor) {
+func setNetMon(lm *netmon.Monitor) {
 }
 
 func debugInfo() string {