.po files from nightly tarballs

 I have no plans to stop occasionally refreshing .po files at
http://files.freeciv.org/translations, but you also have a new option
to get them. We generate nightly source tarballs of each branch, and
.po files are part of that tarball. Tarball generation should also
refresh them against the latest English originals.

 These tarballs are available, along some other files, from:
http://files.freeciv.org/packages/windows/testing/cazfi/installer_cross/nightly/


 - ML