luci-app-ddns - customer dns provider config defaults to 3322.org

[mavack]

First raised in #4150 which resolved custom ddns not completely working, however this issue still remains.

Issue
Create new Custom DDNS provider
Save the config
go back to the ddns screen
click edit
The displayed configuration now defaults to provider showing 3322.org (first in the list) you need to change it back to custom before save.

I believe the problem is here, A custom DNS provider does not have a defined service_name UCI attribute as such it cannot return/display "-- custom --"

o.cfgvalue = function(section_id) {
	return uci.get('ddns', section_id, 'service_name');
}

The "-- custom --" variable is added to the list manually and is not defined as a service_name in uci config.
o.value('-',"-- " + _("custom") + " --");

Need to if uci.get return nothing then display custom. This may or may not work as not sure if there are other exceptions that could be broken by returning custom only when no service_name is found.