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/sock/connect_test.c

@@ -0,0 +1,85 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/atomic.h"
+#include "libc/calls/calls.h"
+#include "libc/dce.h"
+#include "libc/errno.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/runtime/runtime.h"
+#include "libc/sock/sock.h"
+#include "libc/sock/struct/pollfd.h"
+#include "libc/sock/struct/sockaddr.h"
+#include "libc/sysv/consts/af.h"
+#include "libc/sysv/consts/ipproto.h"
+#include "libc/sysv/consts/limits.h"
+#include "libc/sysv/consts/poll.h"
+#include "libc/sysv/consts/sock.h"
+#include "libc/testlib/subprocess.h"
+#include "libc/testlib/testlib.h"
+#include "libc/thread/thread.h"
+
+TEST(connect, nonblocking) {
+  if (IsFreebsd()) return;  // TODO(jart): why did this start flaking?
+  char buf[16] = {0};
+  atomic_uint *sem = _mapshared(sizeof(unsigned));
+  uint32_t addrsize = sizeof(struct sockaddr_in);
+  struct sockaddr_in addr = {
+      .sin_family = AF_INET,
+      .sin_addr.s_addr = htonl(0x7f000001),
+  };
+  ASSERT_SYS(0, 3, socket(AF_INET, SOCK_STREAM, IPPROTO_TCP));
+  ASSERT_SYS(0, 0, bind(3, (struct sockaddr *)&addr, sizeof(addr)));
+  ASSERT_SYS(0, 0, getsockname(3, (struct sockaddr *)&addr, &addrsize));
+  ASSERT_SYS(0, 0, listen(3, SOMAXCONN));
+  SPAWN(fork);
+  while (!*sem) pthread_yield();
+  ASSERT_SYS(0, 4, accept(3, (struct sockaddr *)&addr, &addrsize));
+  ASSERT_SYS(0, 2, read(4, buf, 16));  // hi
+  ASSERT_SYS(0, 5, write(4, "hello", 5));
+  ASSERT_SYS(0, 3, read(4, buf, 16));  // bye
+  PARENT();
+  ASSERT_SYS(0, 0, close(3));
+  ASSERT_SYS(0, 3, socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, IPPROTO_TCP));
+  ASSERT_SYS(EINPROGRESS, -1,
+             connect(3, (struct sockaddr *)&addr, sizeof(addr)));
+  if (!(IsLinux() || IsNetbsd())) {
+    // this doens't work on rhel7 and netbsd
+    ASSERT_SYS(EALREADY, -1,
+               connect(3, (struct sockaddr *)&addr, sizeof(addr)));
+  }
+  ASSERT_SYS(EAGAIN, -1, read(3, buf, 16));
+  *sem = 1;
+  {  // wait until connected
+    struct pollfd pfd = {3, POLLOUT};
+    ASSERT_SYS(0, 1, poll(&pfd, 1, -1));
+    ASSERT_TRUE(!!(POLLOUT & pfd.revents));
+  }
+  ASSERT_SYS(0, 2, write(3, "hi", 2));
+  {  // wait for other process to send us stuff
+    struct pollfd pfd = {3, POLLIN};
+    ASSERT_SYS(0, 1, poll(&pfd, 1, -1));
+    ASSERT_TRUE(!!(POLLIN & pfd.revents));
+  }
+  ASSERT_SYS(0, 5, read(3, buf, 16));
+  ASSERT_STREQ("hello", buf);
+  ASSERT_SYS(0, 3, write(3, "bye", 3));
+  ASSERT_SYS(0, 0, close(3));
+  WAIT(exit, 0);
+  munmap(sem, sizeof(unsigned));
+}