[arch-general] Repo mirrors

[Florian Pritz]

I've changed the mirror script from the wiki so mirror admins can run it every 
minute without huge additional load. This will help a lot to reduce update delays
now and even further when we have multiple tiers (don't know how mucht that works already).

I've already described it somewhere, but for those who don't know it yet:
The script fetches an md5 sum of checksums of the repos (core, extra, testing, community).
If the first checksum is different then the one locally generated on the mirror, it will 
check those for every repo. If they are also different it will rsync as needed. 

HTTP requests are used to fetch the checksums because most mirrors already have HTTP servers
running and because they are cheap. 

I've also created a patch for db-scripts to generate the md5sums for the master server. 
As I don't have a repo set up this patch is untested (it's the same code as in the 
mirrorscript though).

Once this is merged on the master we can start to tell mirror admins to use the new mirrorscript 
which can be found at http://karif.server-speed.net/~flo/tmp/mirrorsync.sh.txt

C&C welcome :)