SELinux compliance

The backend should now work with SELinux enabled.

I did two things to make it work:
* set *httpd_sys_content_t* as type for the cvmfs repository directories in **/cvmfs/[repo name]**
* set *system_u:object_r:default_t:s0* as the context for aufs mount points in **/etc/fstab** in order to allow access to normal users