Download sources from servers that don't respond to ping

[Jann Röšder]

> MacPorts 1.7 changed this to ping all download servers and try them in
> ping order. If a server does not respond to ping or is offline, it
> will be at the bottom of the list and will be tried last.

I have seen an instance where the macports mirror servers were tried
which did not have the file. The server specified in the port file was
not tried at all. For some reason even though the macports mirror
servers didn't have the file a file with size 0 was created, thus
leading to a hash mismatch.

Obviously this behavior is not good.