Do not generate useless argument arrays in whd_* functions.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16954 85f007b7-540e-0410-9357-904b9bb8a0f7
Useless array-to-list casts in Unification.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16955 85f007b7-540e-0410-9357-904b9bb8a0f7