Solve various issues

- #1: use --prefix instead of --local. My previous problem was due to
   the fact that I never did make install !

- #2:  implemented an option for list, that shows all existing Coq
   versions

- #3: use an option to specify whether all existing packages should be
   reinstalled

- #4: use pin in the freshly created switch, in order to avoid
   spurious upgrade to coq.HoTT...