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/calls/fstatat-sysv.c

@@ -28,7 +28,6 @@
  */
 int32_t sys_fstatat(int32_t dirfd, const char *path, struct stat *st,
                     int32_t flags) {
-  int rc;
   void *p;
   union metastat ms;
   if (IsAsan() && !__asan_is_valid_str(path)) return efault();