win/tcp: don't enable iocp sync bypass when iocp emulation is used

When iocp sync bypass is in use libuv doesn't expect the system to
generate events when an i/o operation completes synchronously. However
when iocp emulation is enabled an event will always be generated because
SetFileCompletionNotificationModes() doesn't stop OVERLAPPED.hEvent from
becoming signaled.

This should fix joyent/node#4959.