Prefer memset to repeatedly assigning Qnil

Prefer memset to repeatedly assigning Qnil

* alloc.c (allocate_pseudovector): Catch more bogus values.
* alloc.c (allocate_pseudovector):
* callint.c (Fcall_interactively):
* coding.c (syms_of_coding):
* fringe.c (init_fringe):
Verify that Qnil == 0.
* callint.c (Fcall_interactively):
* eval.c (Fapply, Ffuncall):
* fns.c (mapcar1, larger_vector):
* font.c (font_expand_wildcards):
* fringe.c (init_fringe):
Prefer memset to assigning zeros by hand.
* callint.c (Fcall_interactively):
Remove duplicate assignment of Qnil to args[i].
* coding.c (syms_of_coding):
Prefer LISP_INITIALLY_ZERO to assigning zeros by hand.
* fileio.c (Ffile_selinux_context):
Rewrite to avoid need for Lisp_Object array.
* lisp.h (XLI_BUILTIN_LISPSYM): New macro.
(DEFINE_LISP_SYMBOL_END): Use it.
(NIL_IS_ZERO): New constant.
(memsetnil): New function.