pcap_dump_file(3) on FreeBSD

The text a6862b9f0e7b, as FreeBSD 8.4-RELEASE, FreeBSD 8.3-RELEASE, FreeBSD 8.2-RELEASE, FreeBSD 8.1-RELEASE, FreeBSD 8.0-RELEASE shipped it.

get the standard I/O stream for a savefile being written