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.

third_party/nsync/mu_semaphore.c

@@ -23,7 +23,7 @@
 /* Apple's ulock (part by Cosmo futexes) is an internal API, but:
    1. Unlike GCD it's cancellable, i.e. can be EINTR'd by signals
    2. We currently always use ulock anyway for joining threads */
-#define PREFER_GCD_OVER_ULOCK 0
+#define PREFER_GCD_OVER_ULOCK 1
 
 asm(".ident\t\"\\n\\n\
 *NSYNC (Apache 2.0)\\n\