[#3831] Initial impl of restricted ctl sockets

[#3831] Initial impl of restricted ctl sockets

Working, have some UTs that still need to be fixed

/doc/examples/kea4/advanced.json
/doc/examples/kea4/all-keys-netconf.json
/doc/examples/kea4/all-keys-netconf.json
/doc/examples/kea4/all-keys.json
/doc/examples/kea4/comments.json
/doc/examples/kea4/config-backend.json
/doc/examples/kea4/ha-load-balancing-server1-mt-with-tls.json
/doc/examples/kea4/ha-load-balancing-server2-mt.json
/doc/examples/kea6/advanced.json
/doc/examples/kea6/all-keys-netconf.json
/doc/examples/kea6/all-keys.json
/doc/examples/kea6/comments.json
/doc/examples/kea6/config-backend.json
/doc/examples/kea6/ha-hot-standby-server1-with-tls.json
/doc/examples/kea6/ha-hot-standby-server2.json
    removed /tmp path from socket-name

/src/bin/dhcp4/tests/config_parser_unittest.cc
/src/bin/dhcp4/tests/ctrl_dhcp4_srv_unittest.cc
/src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
/src/bin/dhcp4/tests/dhcp4_test_utils.cc
/src/bin/dhcp4/tests/dhcp4_test_utils.h
/src/bin/dhcp6/tests/config_parser_unittest.cc
/src/bin/dhcp6/tests/ctrl_dhcp6_srv_unittest.cc
/src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
/src/bin/dhcp6/tests/dhcp6_test_utils.cc
/src/bin/dhcp6/tests/dhcp6_test_utils.h
    updated tests

/src/lib/config/Makefile.am
/src/lib/config/meson.build
    defined CONTROL_SOCKET_DIR

/src/lib/config/tests/unix_command_config_unittests.cc
/src/lib/config/tests/unix_command_mgr_unittests.cc
    updated tests

/src/lib/config/unix_command_config.*
    UnixCommandConfig - added PathChecker singleton and methods
    to set and validate socket path/permissions

/src/lib/util/filesystem.*
    Added getPermsissions() and hasPermsission()

/src/lib/util/tests/filesystem_unittests.cc
    new permissions tests