Revert "Only arguments declared as implicit can be renamed"

This reverts commit 18a89c271d540d794d44e41f50587bb1cad69802.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16640 85f007b7-540e-0410-9357-904b9bb8a0f7
better error message for unexpected renaming (closes #2987)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16641 85f007b7-540e-0410-9357-904b9bb8a0f7