FIX: Set SELinux Context of /var/lib/cvmfs after Installing

This adds `restorecon -R /var/lib/cvmfs` to the post-install script of the RPM package (if SELinux handling is enabled). Otherwise the `/var/lib/cvmfs` directory does not pick up the context defined in `cvmfs.fc`.

**Caveat:** (to be fixed) there should be a mechanism to invoke `restorecon` when updating CVMFS and cache directories have been relocated by the administrator. Where would be the best place to do that?