Subtyping: align coqtop behavior concerning opaque csts on coqchk's one

 After discussion with Bruno and Hugo, coqtop now accepts that an opaque
 constant in a module type could be implemented by anything of
 the right type, even if bodies differ. Said otherwise, with respect
 to subtyping, an opaque constant behaves just as a parameter.
 This was already the case in coqchk, and a footnote in documentation
 is advertising for quite some time that:

   "Opaque definitions are processed as assumptions."

 Truly, it might seem awkward that "Definition x:=3" can implement
 "Lemma x:nat. Proof 2. Qed." but the opacity ensures that nothing
 can go wrong afterwards, since Coq is forced to ignore that the x
 in signature has body "2".

 Similarly, "T with Definition x := c" is now legal when T contains
 an opaque x, even when this x isn't convertible with c.

 By avoiding accesses to opaque bodies, we also achieve some speedup
 (less delayed load of .vo final sections containing opaque terms).

 Nota: the extraction will have to be adapted, since for the moment it
 might access the body of opaque constants: the warning emitted when
 doing that should become an error.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@13987 85f007b7-540e-0410-9357-904b9bb8a0f7

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