net/netmon, add: add netmon.State type alias of interfaces.State

... in prep for merging the net/interfaces package into net/netmon.

This is a no-op change that updates a bunch of the API signatures ahead of
a future change to actually move things (and remove the type alias)

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

Change-Id: I477613388f09389214db0d77ccf24a65bff2199c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/netcheck_test.go

@@ -18,7 +18,6 @@ import (
 	"testing"
 	"time"
 
-	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/stun"
 	"tailscale.com/net/stun/stuntest"
@@ -673,7 +672,7 @@ func TestMakeProbePlan(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			ifState := &interfaces.State{
+			ifState := &netmon.State{
 				HaveV6: tt.have6if,
 				HaveV4: !tt.no4,
 			}