poll(): ifndef HAVE_POLL means we don't use poll(), not that it doesn't exist. In OSX poll() is broken for sockets, so we don't use it. Since poll is a kernel function in OSX however, the kernel implementation was used whenever we used poll(), instead of VLCs private poll() which was the intended behaviour. Renamed poll() to vlc_poll() to prevent this.

This fixes #1687, receiving SAPs on Mac OS X.