Better isolate code that assumes NIL_IS_ZERO

Suggested by Stefan Monnier in:
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00588.html
* alloc.c (allocate_pseudovector):
Use memclear, not memsetnil, to remove a 'verify'.
* callint.c (Fcall_interactively):
* dispnew.c (realloc_glyph_pool):
* xdisp.c (init_iterator):
Use memclear, not memset, to remove a 'verify'.
* lisp.h (memclear): Rename from memsetnil, and take a byte
count rather than a word count.  All callers changed.

src/coding.c

@@ -11273,7 +11273,7 @@ internal character representation.  */);
 
   {
     Lisp_Object args[coding_arg_undecided_max];
-    memsetnil (args, ARRAYELTS (args));
+    memclear (args, sizeof args);
 
     Lisp_Object plist[16];
     plist[0] = intern_c_string (":name");