poll-loop: Port to Windows.

poll-loop: Port to Windows.

Use WaitForMultipleObjects for polling on windows.  This works on all kinds
of objects, e.g. sockets, files, especially ioctl calls to the kernel.
poll_fd_wait_event() is used if events need to be passed to pollfds.  latch
is signaled with event, to be waited/polled by WaitForMultipleObjects() as
well.  Changed array of fds to hmap to check for duplicate fds.

Signed-off-by: Linda Sun <lsun@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>