When activating a device, return PCAP_ERROR_IFACE_NOT_UP if the device

When activating a device, return PCAP_ERROR_IFACE_NOT_UP if the device
isn't up, so applications can report that differently from a generic
error (the latter could mean there's a bug somewhere in libpcap).

When capturing on a device without mmap on Linux, ignore ENETDOWN, so
that we can continue to capture traffic if the interface goes down and
comes back up again; comments in the kernel indicate that we'll just
block waiting for packets if we try to receive from a socket that
delivered ENETDOWN, and, if we're using a memory-mapped buffer, we won't
even get notified of "network down" events.