Fix iterating over sub-folders
Fix iterating over sub-folders

This is necessary for situation such as:
~/.mail
   inbox
      cur
      new
      tmp
      maillists
      	cur
      	new
      	tmp
      	mailman
      	  cur
      	  new
      	  tmp

With the former approach the iteration was stopped as it assumed there
was no sub-folder within the maildir folder, but that can actually
happen and with this change, findmaildirs supports this.

AMDmi3/findmaildirs