Have a pcap_if_list_t structure for use by the findalldevs code.

Have a pcap_if_list_t structure for use by the findalldevs code.

It's not part of the API, but it's an internal structure used by the
findalldevs code.  Currently, it just has a pointer to the beginning of
the list, but it could change in order to speed up the process of adding
to the list.