system: add pluginctl -s support
system: add pluginctl -s support

For legacy components route -s option through plugins_services()
to get a list of services that can be controlled like the GUI
controls.  E.g.:

    # pluginctl dhcpd [start|stop|restart]

PR: https://forum.opnsense.org/index.php?topic=12781.0

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core