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.

examples/nanosleep_test.c

@@ -18,7 +18,7 @@
 
 int main(int argc, char *argv[]) {
   long i, ns;
-  struct timespec x, y, w;
+  struct timespec x, y;
   timespec_sleep(timespec_fromnanos(0));  // warmup
 
   printf("\nrelative sleep\n");