Removing the last global evar generation in Term_dnet. The very
fact that the previous implementation was working is still black
magic to me, as evars may have collided in intriguing ways...

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16785 85f007b7-540e-0410-9357-904b9bb8a0f7