Added a AdminMonitor to to warn when this subsystem is disabled

... unless a few conditions indicate that it is likely irrelevant.

core/src/main/java/jenkins/security/admin/MasterKillSwitchWarning.java

@@ -0,0 +1,43 @@
+package jenkins.security.admin;
+
+import hudson.Extension;
+import hudson.model.AdministrativeMonitor;
+import jenkins.model.Jenkins;
+import org.kohsuke.stapler.HttpResponse;
+import org.kohsuke.stapler.HttpResponses;
+import org.kohsuke.stapler.QueryParameter;
+
+import javax.inject.Inject;
+import java.io.IOException;
+
+/**
+ * If {@link AdminWhitelistRule#masterKillSwitch} is on, warn the user.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.THU
+ */
+@Extension
+public class MasterKillSwitchWarning extends AdministrativeMonitor {
+    @Inject
+    AdminWhitelistRule rule;
+
+    @Inject
+    Jenkins jenkins;
+
+    @Override
+    public boolean isActivated() {
+        return rule.getMasterKillSwitch()
+            && jenkins.getComputers().length>1  // if there's no slave, there's no point
+            && jenkins.isUseSecurity()          // if security is off, likewise this is pointless
+            ;
+    }
+
+    public HttpResponse doAct(@QueryParameter String dismiss) throws IOException {
+        if(dismiss!=null) {
+            disable(true);
+            return HttpResponses.redirectViaContextPath("/manage");
+        } else {
+            return HttpResponses.redirectViaContextPath("configureSecurity");
+        }
+    }
+}