LibUV: Implement LibuvEventPoll

LibUV event will use the best available polling method on a system, kind
of like aria2 does already with the different *EventPoll
implementations.
However, libuv may support different/newer polling mechanisms; for
example on Windows it will use IO Completion Ports which are superior to
select() ;)
LibUV: Correct event removal
LibUV: Code cleanup
Libuv: Proper configure detection
LibUV: Reorder event poll preference

*nix will keep epoll/kqueue/port as the default (when available), while
Windows, lacking all of these, will default to libuv (when available)
LibUV: Revert unrelated change
Merge branch 'libuv-eventpoll' of https://github.com/nmaier/aria2 into nmaier-libuv-eventpoll