changed the index derivation machinery to shorten symbol names; use mpl::na instead of null_arg; added #pragma once

[SVN r27833]

libs/multi_index

@@ -1 +1 @@
-Subproject commit 5ebbd58c7ece1dd7fea3d6714ce08e43e4c0e798
+Subproject commit 4809e1efddd263362de014c61ff5ad4b19a05178