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/fadvise.c

@@ -43,7 +43,7 @@ int sys_fadvise_netbsd(int, int, int64_t, int64_t, int) asm("sys_fadvise");
  * @raise ENOSYS on XNU and OpenBSD
  */
 int fadvise(int fd, uint64_t offset, uint64_t len, int advice) {
-  int rc, e = errno;
+  int rc;
   if (IsLinux()) {
     rc = sys_fadvise(fd, offset, len, advice);
   } else if (IsFreebsd() || IsNetbsd()) {