libcurl on interix should not use poll()

on interix, libcurl works fine, as long as it does not use poll(). the problem is, that interix does provide poll(), but the implementing developer must have been in a bad mood, because poll() only works on the /proc filesystem here... *argh*.

a simple patch makes things work again: set disallow_poll=ye for interix too. i attached a patch for it. would be great if you could accept it, i guess it won't do any harm ;)