[master] Merge branch 'trac5032' (mac-sources, control-socket, relay parsers)

# Conflicts:
#	src/lib/testutils/io_utils.cc
#	src/lib/testutils/io_utils.h

diff --cc doc/guide/dhcp4-srv.xml
Simple merge

diff --cc doc/guide/dhcp6-srv.xml
Simple merge

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

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

diff --cc src/lib/testutils/io_utils.cc
Simple merge

diff --cc src/lib/testutils/io_utils.h
Simple merge