MFC r313695, r313760, r314769, r314863, r314865, r316125

r313695:
MFV r313676: libpcap 1.8.1

r313760:
MFV r313759: license change for a few headers (4 clause BSD to 3 clause BSD).

X-MFC-with: r313695

r314769:
Remove compatibility with old libpcap.

Differential Revision: https://reviews.freebsd.org/D9606

r314863:
Stop installing pcap-int.h, which is the internal interface for libpcap.

Reference: https://github.com/the-tcpdump-group/libpcap/issues/560
PR: 217221

r314865:
Bump __FreeBSD_version for removal of pcap-int.h.

PR: 217221

r316125:
MFV r316124: Fix build when WITHOUT_INET6.

Reported by: Randy Westlund <rwestlun gmail com>