dpif-linux: Use "epoll" instead of poll().

dpif-linux: Use "epoll" instead of poll().

epoll appears to be much more efficient than poll() at least for
static file descriptor sets.  I can't otherwise explain why this
patch increases netperf CRR performance by 20% above the previous
commit, which is also about a 19% overall improvement versus
the baseline from before the poll_fd_woke() optimization was
removed.