Make important improvements

- Fix preadv() and pwritev() for old distros
- Introduce _npassert() and _unassert() macros
- Prove that file locks work properly on Windows
- Support fcntl(F_DUPFD_CLOEXEC) on more systems

libc/stdio/flockfile.c

@@ -23,5 +23,5 @@
  * Acquires reentrant lock on stdio object, blocking if needed.
  */
 void(flockfile)(FILE *f) {
-  pthread_mutex_lock(&f->lock);
+  pthread_mutex_lock((pthread_mutex_t *)f->lock);
 }