Unify backup write logic

Use the same backup write helper function for both periodic
background backups and for temporary backups in safeWrite().

Besides just removing code duplication, this brings the advantages
of both together:

- Temporary backups in safeWrite() now use the same atomic mechanism
  when replacing an already existing backup. So that if micro crashes
  in the middle of writing the backup in safeWrite(), this corrupted
  backup will not overwrite a previous good backup.

- Better error handling for periodic backups.

internal/buffer/save.go

@@ -333,27 +333,6 @@ func (b *Buffer) saveToFile(filename string, withSudo bool, autoSave bool) error
 	return err
 }
 
-func (b *SharedBuffer) writeBackup(path string) (string, error) {
-	backupDir := b.backupDir()
-	if _, err := os.Stat(backupDir); err != nil {
-		if !errors.Is(err, fs.ErrNotExist) {
-			return "", err
-		}
-		if err = os.Mkdir(backupDir, os.ModePerm); err != nil {
-			return "", err
-		}
-	}
-
-	backupName := util.DetermineEscapePath(backupDir, path)
-	_, err := b.overwriteFile(backupName)
-	if err != nil {
-		os.Remove(backupName)
-		return "", err
-	}
-
-	return backupName, nil
-}
-
 // safeWrite writes the buffer to a file in a "safe" way, preventing loss of the
 // contents of the file if it fails to write the new contents.
 // This means that the file is not overwritten directly but by writing to the