genapp: Difficult to change and set proper socket filename.

[clarsonneur]

forjj through goforjj use <plugin>.yaml: /plugin: <value> to connect to the proper socket.

If we created a new plugin with a wrong name, the cli.go is generated and will never be updated if we do not remove it, before a go generate

So, even if we change <plugin>.yaml, forjj will take care of it, but not the plugin until we change manually the cli.go in load_plugin_def function

We need forjj and plugins to be more coherent.
The rule should be:

• if <plugin>.yaml: /runtime/serviice/socket:<value> is set, use it in both forjj and the plugin.
• if not set, use <plugin>.yaml: /plugin: <value> in both forjj and the plugin.