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/sock/closesocket-nt.c

@@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/intrin/weaken.h"
 #include "libc/mem/mem.h"
 #include "libc/nt/thunk/msabi.h"
 #include "libc/nt/winsock.h"
@@ -30,9 +31,11 @@ __msabi extern typeof(__sys_closesocket_nt) *const __imp_closesocket;
  *
  * This function should only be called by close().
  */
-textwindows int sys_closesocket_nt(struct Fd *fd) {
-  int rc = __imp_closesocket(fd->handle);
-  if (rc != -1) {
+textwindows int sys_closesocket_nt(struct Fd *f) {
+  if (_weaken(sys_connect_nt_cleanup)) {
+    _weaken(sys_connect_nt_cleanup)(f, true);
+  }
+  if (!__imp_closesocket(f->handle)) {
     return 0;
   } else {
     return __winsockerr();