Revert "Only arguments declared as implicit can be renamed"

This reverts commit 071829b388f66bff8c27749608e1f28c6f6b1dcb.

This policy is too strict, going back to the old behavior

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16638 85f007b7-540e-0410-9357-904b9bb8a0f7
better error message for unexpected renaming (closes #2987)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16639 85f007b7-540e-0410-9357-904b9bb8a0f7