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>

sysdeps/unix/sysv/linux/sigwait.c

@@ -25,7 +25,7 @@ __sigwait (const sigset_t *set, int *sig)
   siginfo_t si;
   int ret;
   do
-    ret = __sigtimedwait (set, &si, 0);
+    ret = __sigtimedwait (set, &si, NULL);
   /* Applications do not expect sigwait to return with EINTR, and the
      error code is not specified by POSIX.  */
   while (ret < 0 && errno == EINTR);