check for traces of CernVM-FS 2.0.x
add cvmfs-server < 2.1 as conflict into RPM
add cvmfs-server < 2.1 as conflict into DEB
Merge pull request #407 from reneme/feature-check_legacy

Feature: Check Presence of Traces of Legacy CernVM-FS 2.0.x

This checks for typical traces of the CernVM-FS 2.0.x server tools and refuses to do anything in cvmfs_server until they are removed. Furthermore it defines any cvmfs-server package older than 2.1.x as a conflict in both RPM and Deb packages.