Improve threading and i/o routines

- On Windows connect() can now be interrupted by a signal; connect() w/
  O_NONBLOCK will now raise EINPROGRESS; and connect() with SO_SNDTIMEO
  will raise ETIMEDOUT after the interval has elapsed.

- We now get the AcceptEx(), ConnectEx(), and TransmitFile() functions
  from the WIN32 API the officially blessed way, using WSAIoctl().

- Do nothing on Windows when fsync() is called on a directory handle.
  This was raising EACCES earlier becaues GENERIC_WRITE is required on
  the handle. It's possible to FlushFileBuffers() a directory handle if
  it's opened with write access but MSDN doesn't document what it does.
  If you have any idea, please let us know!

- Prefer manual reset event objects for read() and write() on Windows.

- Do some code cleanup on our dlmalloc customizations.

- Fix errno type error in Windows blocking routines.

- Make the futex polyfill simpler and faster.

libc/calls/releasefd.c

@@ -25,6 +25,7 @@
 void __releasefd(int fd) {
   int f1, f2;
   if (!(0 <= fd && fd < g_fds.n)) return;
+  g_fds.p[fd].kind = kFdEmpty;
   bzero(g_fds.p + fd, sizeof(*g_fds.p));
   f1 = atomic_load_explicit(&g_fds.f, memory_order_relaxed);
   do {