pkt_op: favor level-triggered epoll for fairness

pkt_op: favor level-triggered epoll for fairness

Sigfd->event_step needs priority over PktOp (and everything else).
We'll also add ECONNRESET checking, here, since it could see
bidirectional use in the future.

This is unlikely to have any sort of performance difference
since this is only for small, occasional packets, but the code
reduction is nice.