[CentOS-mirror] Some disabled mirrors and CentOS 7.4.1708

Hi, I just disabled 27 mirrors that had been unreachable since our 
mirror monitoring node move a few months ago, and those mirrors that 
were last synced more than half a year ago. If you find that your mirror 
has disappeared from http://mirror-status.centos.org/ and wish to be 
reinstated, please let me know. Note that if you are no longer listed as 
a mirror, your mirror's IP addresses have also been removed from the 
msync.centos.org ACL, so you may need to re-sync from some other 
external mirror first.

There were also 18 rsync URLs and 12 FTP URLs that had been inaccessible 
for the last 7 months. Those were also removed from the list, leaving 
the other URLs for each mirror intact. This change will help users (and 
new mirror admins) in finding a working rsync URL for their rsyncing needs.

In other happier news, CentOS 7.4.1708 is approaching. You already have 
nearly all of 7.4.1708's packages because those were shipped in 
7.3.1611's CR repository. Using rsync's -H option (preserve hard links) 
will simply link the 7.3.1611 CR and 7.4.1708 base packages together 
when 7.4.1708 is released to mirrors, saving quite a lot of bandwidth. 
Obviously the hard linking will not help with the 16GB of 7.4.1708 .iso 
images. The old 7.3.1611 release will be removed from mirrors later on.