backup: extract StdioWrapper from ProgressPrinters

The StdioWrapper is not used at all by the ProgressPrinters. It is
called a bit earlier than previously. However, as the password prompt
directly accessed stdin/stdout this doesn't cause problems.

internal/ui/backup/progress_test.go

@@ -2,7 +2,6 @@ package backup
 
 import (
 	"context"
-	"io"
 	"sync"
 	"testing"
 	"time"
@@ -45,9 +44,6 @@ func (p *mockPrinter) Finish(id restic.ID, _ time.Time, summary *Summary, dryRun
 
 func (p *mockPrinter) Reset() {}
 
-func (p *mockPrinter) Stdout() io.WriteCloser { return nil }
-func (p *mockPrinter) Stderr() io.WriteCloser { return nil }
-
 func (p *mockPrinter) P(msg string, args ...interface{}) {}
 func (p *mockPrinter) V(msg string, args ...interface{}) {}