tsnet: do not log using log.Printf by default

Printing to log.Printf by default made sense when there was
no automatic uploading to log.tailscale.io.
However, now that we support that by default,
logging to log.Printf if Server.Logf is nil seems superfluous.

The tsnet package is also intended to be instantiated as a library.
Thus, a single process can multiple tsnet instances running simultaneously.
The default behavior of each logging to log.Printf makes the
os.Stderr  of the main process unreadable.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

tsnet/tsnet.go

@@ -11,7 +11,6 @@ import (
 	"context"
 	"fmt"
 	"io/ioutil"
-	"log"
 	"net"
 	"net/http"
 	"os"
@@ -62,8 +61,8 @@ type Server struct {
 	// If empty, the binary name is used.
 	Hostname string
 
-	// Logf, if non-nil, specifies the logger to use. By default,
-	// log.Printf is used.
+	// Logf, if non-nil, specifies an additional logger to use such that
+	// logs are both written here and uploaded to log.tailscale.io.
 	Logf logger.Logf
 
 	// Ephemeral, if true, specifies that the instance should register
@@ -331,9 +330,7 @@ func (s *Server) logf(format string, a ...interface{}) {
 	}
 	if s.Logf != nil {
 		s.Logf(format, a...)
-		return
 	}
-	log.Printf(format, a...)
 }
 
 // printAuthURLLoop loops once every few seconds while the server is still running and