getting statistics for a pcap_t. Have "pcap_stats()" call it, rather
than being a per-platform function; have stats routines for non-live
pcap_t's that return an error.
the platform-dependent part of closing a pcap_t (and the
live-vs-savefile part as well, so that function must close the file
descriptor and free up any buffers allocated).
In the Digital UNIX support, add in a check for a memory allocation
failure.