[master] DDNS parameter, replace-client-name, now supports multiple modes

    Merges in trac4259.

diff --cc src/bin/dhcp4/dhcp4_srv.cc
Simple merge

diff --cc src/bin/dhcp4/tests/config_parser_unittest.cc
Simple merge

diff --cc src/bin/dhcp6/dhcp6_srv.cc
Simple merge

diff --cc src/lib/dhcpsrv/tests/d2_udp_unittest.cc
Simple merge