Fix #3001 (rename arg of global cst inside section)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16426 85f007b7-540e-0410-9357-904b9bb8a0f7
Only arguments declared as implicit can be renamed

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16427 85f007b7-540e-0410-9357-904b9bb8a0f7