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/netns/netns_linux.go

@@ -14,7 +14,6 @@ import (
 
 	"golang.org/x/sys/unix"
 	"tailscale.com/envknob"
-	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netmon"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/linuxfw"
@@ -119,7 +118,7 @@ func setBypassMark(fd uintptr) error {
 }
 
 func bindToDevice(fd uintptr) error {
-	ifc, err := interfaces.DefaultRouteInterface()
+	ifc, err := netmon.DefaultRouteInterface()
 	if err != nil {
 		// Make sure we bind to *some* interface,
 		// or we could get a routing loop.