eloop: optimise the pselect code so it's not a wrapper for ppoll

eloop: optimise the pselect code so it's not a wrapper for ppoll

This makes the code smaller yet and also use less memory then ppoll!
Still, the API blows chunks and we still have arbitary fd limits
which we'll realistically never hit.
Also, some BSD's note potential issues with select on the same fd
across processes so ppoll is still the winner.