Undo port to hypothetical nonzero Qnil case

Undo port to hypothetical nonzero Qnil case

This mostly undoes the previous change in this area.  See:
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00570.html
* alloc.c (allocate_pseudovector):
* callint.c (Fcall_interactively):
* dispnew.c (realloc_glyph_pool):
* fringe.c (init_fringe):
* lisp.h (memsetnil):
* xdisp.c (init_iterator):
Simplify by assuming that Qnil is zero, but verify the assumption.
* lisp.h (NIL_IS_ZERO): Revert back to this symbol, removing
NIL_IS_NONZERO.  All uses changed.