Port to hypothetical case where Qnil is nonzero

Port to hypothetical case where Qnil is nonzero

* alloc.c (allocate_pseudovector):
* callint.c (Fcall_interactively):
* coding.c (syms_of_coding):
* dispnew.c (realloc_glyph_pool):
* fringe.c (init_fringe):
* lisp.h (memsetnil):
* xdisp.c (init_iterator):
Port to the currently-hypothetical case where Qnil is nonzero.
* dispnew.c (adjust_glyph_matrix): Remove unnecessary verification,
as there are no Lisp_Object values in the data here.
* lisp.h (NIL_IS_NONZERO): New symbol, replacing NIL_IS_ZERO.
All uses changed.  Define only if not already defined, so that one
can debug with -DNIL_IS_NONZERO.
* xdisp.c (init_iterator): Remove unnecessary initializations to 0.