net/{interfaces,netmon}, all: merge net/interfaces package into net/netmon

In prep for most of the package funcs in net/interfaces to become
methods in a long-lived netmon.Monitor that can cache things.  (Many
of the funcs are very heavy to call regularly, whereas the long-lived
netmon.Monitor can subscribe to things from the OS and remember
answers to questions it's asked regularly later)

Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299

Change-Id: Ie4e8dedb70136af2d611b990b865a822cd1797e5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netmon/interfaces_freebsd.go

@@ -0,0 +1,28 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// This might work on other BSDs, but only tested on FreeBSD.
+
+//go:build freebsd
+
+package netmon
+
+import (
+	"syscall"
+
+	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
+)
+
+// fetchRoutingTable calls route.FetchRIB, fetching NET_RT_DUMP.
+func fetchRoutingTable() (rib []byte, err error) {
+	return route.FetchRIB(syscall.AF_UNSPEC, unix.NET_RT_DUMP, 0)
+}
+
+func parseRoutingTable(rib []byte) ([]route.Message, error) {
+	return route.ParseRIB(syscall.NET_RT_IFLIST, rib)
+}
+
+func getDelegatedInterface(ifIndex int) (int, error) {
+	return 0, nil
+}