Option to turn off Systemd integration

So in

Lines 65 to 67 in a7e3d94

    
           if ENV["NOTIFY_SOCKET"] && !Puma.jruby? 
        
             @config.plugins.create('systemd') 
        
           end

we load the Systemd integration if the NOTIFY_SOCKET is set.

It would be great to have the option to opt out of this, say if ENV["PUMA_SKIP_SYSTEMD"] is set.

In my case I'd like this because in one of my background daemons, I run Puma in a thread with Rack::Handler::Puma.run just as an interface to the daemon. However, since the daemon does way more that Puma doesn't know about, I deal with systemd integration elsewhere in that daemon. Currently I'm just monkeypatching the lines above to turn off the automatic Puma integration.

Makes sense to me 👍🏻

	if ENV["NOTIFY_SOCKET"] && !Puma.jruby?
	@config.plugins.create('systemd')
	end