[JENKINS-41684] Ensure that PluginManager.dynamicLoad runs as SYSTEM (#2732)

* [FIXED JENKINS-41684] Ensure that PluginManager.dynamicLoad runs as SYSTEM.
Test plugin source:
package test;
import hudson.Plugin;
import jenkins.model.Jenkins;
public class ThePlugin extends Plugin {
    @Override
    public void postInitialize() throws Exception {
        Jenkins.getInstance().checkPermission(Jenkins.ADMINISTER);
    }
}

* @daniel-beck wants this all reindented.