and using ftell(); that won't necessarily work on Windows (if libpcap
was built with a different version of the C runtime library than tcpdump
is), and, even on UN*X, would break if a pcap_dumper_t * were ever made
something other than a FILE *.
Provide a pcap_dump_ftell() implementation that does that cheating cast
for use if libpcap doesn't have it (a pcap_dumper_t * is just a FILE *
on those older versions of libpcap).
