The empty string is now a recognized configuration filename. It means that
the default configuration with an empty loadpath should be loaded. It also
means it will not be saved.

This removes the need for a WHY3NOCONFIG environment variable and some
tricks based on the WHY3LOADPATH variable. This also removes the need for
a /dev/null file.

the main program is transformed into a test of compute_sums.
It seems that the metas are not needed anymore.

and a bit of cleaning at the same time

  Fix: shortcut for provers which are unknown

Johannes, Yannick: yes this is what you think (good news)
but this is not yet completed (bad news)

(we do not have to use the ghost construct here, but it looks nicer)