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>

cmd/tailscale/cli/status.go

@@ -23,7 +23,7 @@ import (
 	"golang.org/x/net/idna"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
-	"tailscale.com/net/interfaces"
+	"tailscale.com/net/netmon"
 	"tailscale.com/util/dnsname"
 )
 
@@ -102,7 +102,7 @@ func runStatus(ctx context.Context, args []string) error {
 		if err != nil {
 			return err
 		}
-		statusURL := interfaces.HTTPOfListener(ln)
+		statusURL := netmon.HTTPOfListener(ln)
 		printf("Serving Tailscale status at %v ...\n", statusURL)
 		go func() {
 			<-ctx.Done()