[master] Merge branch 'trac3203' (DHCP client classification, part 1)
Conflicts: src/bin/dhcp4/dhcp4_srv.cc src/bin/dhcp4/tests/dhcp4_test_utils.h src/bin/dhcp6/dhcp6_srv.cc src/lib/dhcp/pkt4.cc src/lib/dhcp/tests/pkt4_unittest.cc
Conflicts: src/bin/dhcp4/dhcp4_srv.cc src/bin/dhcp4/tests/dhcp4_test_utils.h src/bin/dhcp6/dhcp6_srv.cc src/lib/dhcp/pkt4.cc src/lib/dhcp/tests/pkt4_unittest.cc