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/netmon/netmon_darwin_test.go

@@ -0,0 +1,27 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package netmon
+
+import (
+	"encoding/hex"
+	"strings"
+	"testing"
+
+	"golang.org/x/net/route"
+)
+
+func TestIssue1416RIB(t *testing.T) {
+	const ribHex = `32 00 05 10 30 00 00 00 00 00 00 00 04 00 00 00 14 12 04 00 06 03 06 00 65 6e 30 ac 87 a3 19 7f 82 00 00 00 0e 12 00 00 00 00 06 00 91 e0 f0 01 00 00`
+	rtmMsg, err := hex.DecodeString(strings.ReplaceAll(ribHex, " ", ""))
+	if err != nil {
+		t.Fatal(err)
+	}
+	msgs, err := route.ParseRIB(route.RIBTypeRoute, rtmMsg)
+	if err != nil {
+		t.Logf("ParseRIB: %v", err)
+		t.Skip("skipping on known failure; see https://github.com/tailscale/tailscale/issues/1416")
+		t.Fatal(err)
+	}
+	t.Logf("Got: %#v", msgs)
+}