Tweak the value returned from Fgarbage_collect again.

Tweak the value returned from Fgarbage_collect again.
* src/alloc.c (Fgarbage_collect): New return value, as confirmed in
http://lists.gnu.org/archive/html/emacs-devel/2012-07/msg00418.html.
Adjust documentation.
(total_vector_bytes): Rename to total_vector_slots, adjust
accounting.
(total_free_vector_bytes): Rename to total_free_vector_slots,
adjust accounting.
(Qstring_bytes, Qvector_slots): New symbols.
(syms_of_alloc): DEFSYM them.
* lisp/emacs-lisp/chart.el (chart-emacs-storage): Adjust again.