1. is_meta_closed should be applied only to terms on which a substitution

1. is_meta_closed should be applied only to terms on which a substitution
   has already been applied. Fixed here and there.
   Note: it is not possible to add a subst parameter to is_meta_closed since
   the function is declared in cic/CicUtil and it is used where the unification
   is not available (e.g. in whelp and library)
2. known (but never verified before) bug fixed: delifting should fail
   (instead of raising Uncertain) iff the local context is not meta_closed.
   The test used to be "does not contain a Meta"