Repository files should not be other-writable

All the files created by cvmfs_swissknife in hosted repositories and replicas are mode 666, that is, writable by "other". It's not a big security problem because if files are tampered with they will not be able to be distributed due to all the secure hashes and signatures, but still, it shouldn't be that way. It is a potential denial of service if one repository administrator tampered with files of another one on a shared repository server.

I tested this with recent source code from git and it is still happening.