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/fdatasync-nt.c

@@ -18,15 +18,24 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
+#include "libc/nt/enum/fileflagandattributes.h"
 #include "libc/nt/enum/filetype.h"
 #include "libc/nt/files.h"
+#include "libc/nt/struct/byhandlefileinformation.h"
 #include "libc/sysv/errfuns.h"
 #ifdef __x86_64__
 
 textwindows int sys_fdatasync_nt(int fd, bool fake) {
+  struct NtByHandleFileInformation wst;
   if (!__isfdopen(fd)) return ebadf();
   if (!__isfdkind(fd, kFdFile)) return einval();
   if (GetFileType(g_fds.p[fd].handle) != kNtFileTypeDisk) return einval();
+  if (!GetFileInformationByHandle(g_fds.p[fd].handle, &wst)) return __winerr();
+  if (wst.dwFileAttributes & kNtFileAttributeDirectory) {
+    // Flushing a directory handle is possible, but it needs
+    // kNtGenericWrite access, and MSDN doesn't document it.
+    return 0;
+  }
   if (_check_cancel() == -1) return -1;
   if (_check_signal(false) == -1) return -1;
   if (fake) return 0;