ipfwpcap: Fix build after libpcap 1.10.6 update

pcap-int.h now references SIZEOF_TIME_T from libpcap's config.h, which
is not available to consumers of the internal header outside of the
libpcap build. Switch to the public <pcap.h> header and replace the
direct FILE* casts and ferror()/fflush() calls with pcap_dump_flush(3),
which is the correct public API for flushing a pcap dump file.

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