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/intrin/ulock.c

@@ -31,7 +31,7 @@
 int sys_ulock_wait(uint32_t operation, void *addr, uint64_t value,
                    uint32_t timeout_micros) asm("sys_futex_cp");
 
-// returns -1 w/ errno
+// returns number of other waiters, or -1 w/ errno
 int ulock_wait(uint32_t operation, void *addr, uint64_t value,
                uint32_t timeout_micros) {
   int rc;