libpcap: Make pcap/bpf.h a wrapper around system net/bpf.h

In the past, we modified pcap/pcap.h to include the system net/bpf.h
rather than libpcap's own pcap/bpf.h. However, starting around 1.10.2,
libpcap requires a few extern functions defined in pcap/bpf.h to build.
Simply reverting that local change and pulling in pcap/bpf.h is not a
solution, because some ports with '#include <pcap.h>' such as mail/spamd
and net/xprobe require the system net/bpf.h to be pulled in. To
accommodate both requirements, make pcap/bpf.h a wrapper around the
system net/bpf.h, but retain the extern function definitions. This is
in preparation for libpcap 1.10.3.

Approved by: cy, philip, emaste, delphij (earlier revision)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38575