tsnet: split user facing and backend logging

This adds a new `UserLogf` field to the `Server` struct.
When set this any logs generated by Server are logged using
`UserLogf` and all spammy backend logs are logged to `Logf`.

If it `UserLogf` is unset, we default to `log.Printf` and
if `Logf` is unset we discard all the spammy logs.

Fixes #12094

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tsnet/example/tsnet-funnel/tsnet-funnel.go

@@ -16,14 +16,12 @@ import (
 	"net/http"
 
 	"tailscale.com/tsnet"
-	"tailscale.com/types/logger"
 )
 
 func main() {
 	flag.Parse()
 	s := &tsnet.Server{
 		Dir:      "./funnel-demo-config",
-		Logf:     logger.Discard,
 		Hostname: "fun",
 	}
 	defer s.Close()