b10-dhcp6 parses and uses ddns-dhcp configuration

Reviewed required a few comments to be added.

b10-dhcp4 now fully able to generate and send DHCP-DDNS requests

Primarily cosmetic changes and commentary.

Reviewed by Thomas Markwalder on Jabber.

Conflicts:
	src/bin/dhcp4/tests/dhcp4_test_utils.h
	src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
	src/bin/dhcp6/tests/dhcp6_test_utils.h

Conflicts:
	src/lib/dns/tests/rdata_nsec3param_like_unittest.cc
	src/lib/dns/tests/rdata_nsecbitmap_unittest.cc