Download script: tighten up md5sums validation.
Work around a reported locale problem.

I've not been able to reproduce the problem here.
Load the RSA key just before it's needed.

This fixes auth problems when connecting to a server after playing locally.
Fix the regexp used for validating the md5sums file.
Fix handling of fully-specified source URLs.
Fail sooner if downloading the md5sum file fails.
If downloading any of the listed files fails, continue with the next one.
Changelog update.