Create the tarball with the libraries and modulefiles from CVMFS directly,
without first copying that content in a scratch directory. This is important
to avoid generating many small files on the local file system, particularly
important if it is Lustre.
As a consequence the options and the code related to the scratch directory
have been removed.
Among the prerequisites the script checks now also that either it has been
called with sudo or with the --fakeroot flag in order to be able to build the
Singularity image.

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