Support to download the mirrors list

[liweitianux]

Currently, the mirrors list is a local file. A first thought, it may be a good idea to let mirrorselect download it from a specified URL. However, after thinking, it seems unnecessary. Updating the local mirrors file isn't necessary harder than updating the remote file pointed by the URL.

Well, if it's a real requirement, we can then do it. By the way, we can then even make mirrorselect to periodically poll for updates.