Since signalfd is often combined with our event loop, give it a
convenient API and reduce the code duplication required to use it.
EventLoop is replaced with ::event_loop to allow consistent
parameter passing and avoid needlessly passing the package name
on stack.
We also avoid exporting SFD_NONBLOCK since it's the only flag we
support. There's no sense in having the memory overhead of a
constant function when it's in cold code.