events: make memory leak warning name more verbose

Switch from a generic `Warning` to the more specific
`MaxListenersExceededWarning`.

Ref: nodejs#8298

doc/api/events.md

@@ -285,6 +285,7 @@ The emitted warning can be inspected with [`process.on('warning')`][] and will
 have the additional `emitter`, `type` and `count` properties, referring to
 the event emitter instance, the event’s name and the number of attached
 listeners, respectively.
+Its `name` property is set to `'MaxListenersExceededWarning'`.
 
 ### emitter.addListener(eventName, listener)
 <!-- YAML

lib/events.js

@@ -259,7 +259,7 @@ function _addListener(target, type, listener, prepend) {
         const w = new Error('Possible EventEmitter memory leak detected. ' +
                             `${existing.length} ${type} listeners added. ` +
                             'Use emitter.setMaxListeners() to increase limit');
-        w.name = 'Warning';
+        w.name = 'MaxListenersExceededWarning';
         w.emitter = target;
         w.type = type;
         w.count = existing.length;

test/parallel/test-event-emitter-max-listeners-warning.js

@@ -11,7 +11,7 @@ e.setMaxListeners(1);
 
 process.on('warning', common.mustCall((warning) => {
   assert.ok(warning instanceof Error);
-  assert.strictEqual(warning.name, 'Warning');
+  assert.strictEqual(warning.name, 'MaxListenersExceededWarning');
   assert.strictEqual(warning.emitter, e);
   assert.strictEqual(warning.count, 2);
   assert.strictEqual(warning.type, 'event-type');