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.

test/libc/stdio/memory_test.c

@@ -28,12 +28,12 @@ void *Worker(void *arg) {
   int i;
   char *volatile p;
   char *volatile q;
-  for (i = 0; i < 256; ++i) {
+  for (i = 0; i < 3000; ++i) {
     p = malloc(17);
     free(p);
     p = malloc(17);
     q = malloc(17);
-    sched_yield();
+    pthread_yield();
     free(p);
     free(q);
   }
@@ -45,7 +45,7 @@ void SetUpOnce(void) {
 }
 
 TEST(memory, test) {
-  int i, n = 32;
+  int i, n = 8;
   pthread_t *t = gc(malloc(sizeof(pthread_t) * n));
   for (i = 0; i < n; ++i) {
     ASSERT_EQ(0, pthread_create(t + i, 0, Worker, 0));