[#3840] Backport ctl channel socket restrictions

[#3840] Backport ctl channel socket restrictions

On branch 3840-restrict-ctl-sockets-v6
Changes to be committed:
	modified:   doc/examples/agent/comments.json
	modified:   doc/examples/agent/simple.json
	modified:   doc/examples/ddns/all-keys-netconf.json
	modified:   doc/examples/ddns/all-keys.json
	modified:   doc/examples/ddns/comments.json
	modified:   doc/examples/ddns/sample1.json
	modified:   doc/examples/ddns/template.json
	modified:   doc/examples/kea4/advanced.json
	modified:   doc/examples/kea4/all-keys-netconf.json
	modified:   doc/examples/kea4/all-keys.json
	modified:   doc/examples/kea4/comments.json
	modified:   doc/examples/kea4/config-backend.json
	modified:   doc/examples/kea4/dhcpv4-over-dhcpv6.json
	modified:   doc/examples/kea4/ha-load-balancing-server1-mt-with-tls.json
	modified:   doc/examples/kea4/ha-load-balancing-server2-mt.json
	modified:   doc/examples/kea6/advanced.json
	modified:   doc/examples/kea6/all-keys-netconf.json
	modified:   doc/examples/kea6/all-keys.json
	modified:   doc/examples/kea6/comments.json
	modified:   doc/examples/kea6/config-backend.json
	modified:   doc/examples/kea6/dhcpv4-over-dhcpv6.json
	modified:   doc/examples/kea6/ha-hot-standby-server1-with-tls.json
	modified:   doc/examples/kea6/ha-hot-standby-server2.json
	modified:   doc/examples/netconf/comments.json
	modified:   doc/examples/netconf/simple-dhcp4.json
	modified:   doc/examples/netconf/simple-dhcp6.json
	modified:   doc/sphinx/arm/agent.rst
	modified:   doc/sphinx/arm/ddns.rst
	modified:   doc/sphinx/arm/dhcp4-srv.rst
	modified:   doc/sphinx/arm/dhcp6-srv.rst
	modified:   doc/sphinx/arm/hooks.rst
	modified:   src/bin/agent/ca_cfg_mgr.cc
	modified:   src/bin/agent/ca_command_mgr.cc
	modified:   src/bin/agent/simple_parser.cc
	modified:   src/bin/agent/tests/ca_cfg_mgr_unittests.cc
	modified:   src/bin/agent/tests/ca_command_mgr_unittests.cc
	modified:   src/bin/agent/tests/ca_controller_unittests.cc
	modified:   src/bin/agent/tests/get_config_unittest.cc
	modified:   src/bin/agent/tests/parser_unittests.cc
	modified:   src/bin/agent/tests/testdata/get_config.json
	modified:   src/bin/d2/tests/d2_cfg_mgr_unittests.cc
	modified:   src/bin/d2/tests/d2_command_unittest.cc
	modified:   src/bin/d2/tests/get_config_unittest.cc
	modified:   src/bin/d2/tests/testdata/get_config.json
	modified:   src/bin/dhcp4/tests/config_parser_unittest.cc
	modified:   src/bin/dhcp4/tests/ctrl_dhcp4_srv_unittest.cc
	modified:   src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
	modified:   src/bin/dhcp4/tests/dhcp4_test_utils.cc
	modified:   src/bin/dhcp4/tests/dhcp4_test_utils.h
	modified:   src/bin/dhcp4/tests/get_config_unittest.cc
	modified:   src/bin/dhcp6/tests/config_parser_unittest.cc
	modified:   src/bin/dhcp6/tests/ctrl_dhcp6_srv_unittest.cc
	modified:   src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
	modified:   src/bin/dhcp6/tests/dhcp6_test_utils.cc
	modified:   src/bin/dhcp6/tests/dhcp6_test_utils.h
	modified:   src/bin/dhcp6/tests/get_config_unittest.cc
	modified:   src/bin/keactrl/kea-ctrl-agent.conf.pre
	modified:   src/bin/keactrl/kea-dhcp-ddns.conf.pre
	modified:   src/bin/keactrl/kea-dhcp4.conf.pre
	modified:   src/bin/keactrl/kea-dhcp6.conf.pre
	modified:   src/bin/keactrl/kea-netconf.conf.pre
	modified:   src/bin/netconf/tests/testdata/get_config.json
	modified:   src/lib/config/Makefile.am
	modified:   src/lib/config/command_mgr.cc
	modified:   src/lib/config/tests/Makefile.am
	modified:   src/lib/config/tests/command_mgr_unittests.cc
	new file:   src/lib/config/tests/unix_command_config_unittests.cc
	new file:   src/lib/config/unix_command_config.cc
	new file:   src/lib/config/unix_command_config.h
	modified:   src/lib/util/filesystem.cc
	modified:   src/lib/util/filesystem.h
	modified:   src/lib/util/tests/filesystem_unittests.cc