Add header for htons(3)

Required for FreeBSD.
Discover if pcap_create(3PCAP) exists

Descend into the autoconf, #ifdef labyrinth by checking for the
existence of pcap_create(). Older versions of libpcap won't have it and
epcap is so simple that it's not much overhead to support them.
Allow setting the pcap buffer size

For versions of pcap that support it, allow increasing the internal pcap
packet buffer for situations where packets are being dropped.

Support for retrieving the packet statistics will be added in the
future.