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/portmapper/portmapper.go

@@ -21,7 +21,6 @@ import (
 
 	"go4.org/mem"
 	"tailscale.com/control/controlknobs"
-	"tailscale.com/net/interfaces"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/neterror"
 	"tailscale.com/net/netmon"
@@ -216,7 +215,7 @@ func NewClient(logf logger.Logf, netMon *netmon.Monitor, debug *DebugKnobs, cont
 	ret := &Client{
 		logf:         logf,
 		netMon:       netMon,
-		ipAndGateway: interfaces.LikelyHomeRouterIP, // TODO(bradfitz): move this to netMon
+		ipAndGateway: netmon.LikelyHomeRouterIP, // TODO(bradfitz): move this to method on netMon
 		onChange:     onChange,
 		controlKnobs: controlKnobs,
 	}