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/tsnet_test.go

@@ -16,6 +16,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"log"
 	"math/big"
 	"net"
 	"net/http"
@@ -214,8 +215,8 @@ func startServer(t *testing.T, ctx context.Context, controlURL, hostname string)
 		Ephemeral:         true,
 		getCertForTesting: testCertRoot.getCert,
 	}
-	if !*verboseNodes {
-		s.Logf = logger.Discard
+	if *verboseNodes {
+		s.Logf = log.Printf
 	}
 	t.Cleanup(func() { s.Close() })