Feature: Custom SELinux Context for CVMFS Cache Directory

After looking deeper into [CVM-644](https://sft.its.cern.ch/jira/browse/CVM-644) and a related problem reported by Oscar Gonzales Lopez, I reconfigured CVMFS's SELinux module to introduce a custom context type. Namely `cvmfs_cache_t` can be used to label the cache location of cvmfs. This should simplify both the debugging and the relocation of the CVMFS cache directory.

Before one would have to label the cache as `var_lib_t` regardless of the actual location of the cache, which seems pretty odd to me.

Thorough tests and verification needed, of course...