Use torrent download script in archlinux package.
Fix Archlinux package cache directory, fix unvanquished.install output.
Archlinux: Remove package source files (PKGBUILD, ChangeLog, *.install).

Since they require a proper tag even for tests, maintaining them in the same
repository is inconvenient. Latest version can always be found in the AUR.
Archlinux: Prepare for unvanquished-data package that puts assets in /usr/share instead of /var/lib.
Merge branch 'archlinux'