Silence most -Wzero-as-null-pointer-constant diagnostics

Replace 0 by NULL and {0} by {}.

Omit a few cases that aren't so trivial to fix.

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>
Link: <https://software.codidact.com/posts/292718/292759#answer-292759>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

support/support_stack_alloc.c

@@ -58,7 +58,7 @@ support_stack_alloc (size_t size)
   /* Use MAP_NORESERVE so that RAM will not be wasted on the guard
      bands; touch all the pages of the actual stack before returning,
      so we know they are allocated.  */
-  void *alloc_base = xmmap (0,
+  void *alloc_base = xmmap (NULL,
                             alloc_size,
                             PROT_NONE,
                             MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE|MAP_STACK,