rc: new ubus object for handling /etc/init.d/ scripts

rc: new ubus object for handling /etc/init.d/ scripts

This commit adds "rc" ubus object with methods "list" and "exec" for
listing and calling init.d script appropriately. It's useful for all
kind of UIs (e.g. LuCI) and custom apps.

Example:
root@OpenWrt:~# ubus call rc list
{
	"blockd": {
		"start": 80,
		"enabled": true,
		"running": true
	},
	"dnsmasq": {
		"start": 19,
		"enabled": true,
		"running": true
	}
}
root@OpenWrt:~# ubus call rc init '{ "name": "blockd", "action": "disable" }'
root@OpenWrt:~# ubus call rc init '{ "name": "dnsmasq", "action": "stop" }'
root@OpenWrt:~# ubus call rc list
{
	"blockd": {
		"start": 80,
		"enabled": false,
		"running": true
	},
	"dnsmasq": {
		"start": 19,
		"enabled": true,
		"running": false
	}
}

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Jo-Philipp Wich <jo@mein.io>