Fix warnings

This change fixes Cosmopolitan so it has fewer opinions about compiler
warnings. The whole repository had to be cleaned up to be buildable in
-Werror -Wall mode. This lets us benefit from things like strict const
checking. Some actual bugs might have been caught too.

libc/intrin/sigcountset.c

@@ -28,7 +28,7 @@
  * @asyncsignalsafe
  */
 int sigcountset(const sigset_t *set) {
-  int r, i, x, y;
+  int x, y;
   switch (_NSIG) {
     case 32:
       x = (uint32_t)set->__bits[0];