When reading from a BPF device, if we get ENXIO, which means "The device

When reading from a BPF device, if we get ENXIO, which means "The device
went down" on at least some OSes, return a message indicating that.

When reading from a Linux PF_PACKET socket, if we get ENETDOWN, which
means "The device went down", return a message indicating that.

When doing a poll() on a PF_PACKET socket, check for various "something
happened on this, but it's not readable" conditions.