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/netmon_test.go

@@ -11,7 +11,6 @@ import (
 	"testing"
 	"time"
 
-	"tailscale.com/net/interfaces"
 	"tailscale.com/util/mak"
 )
 
@@ -116,8 +115,6 @@ func TestMonitorMode(t *testing.T) {
 
 // tests (*State).IsMajorChangeFrom
 func TestIsMajorChangeFrom(t *testing.T) {
-	type State = interfaces.State
-	type Interface = interfaces.Interface
 	tests := []struct {
 		name   string
 		s1, s2 *State