Rearranging here and there the form, in particular simplify the copy
of cvmfs directories into the tarball.
Changes in the content w.r.t. Serhat:
- to get the LD_LIBRARY_PATH parse the output of "alienv printenv"
  directly in the same process not by running a script plus "alienv
  setenv" inside the Singularity image
- add option for building the image with fakeroot