Make the `WebSocket` constructor throw a `SyntaxError` if any of the
subprotocol names are invalid or duplicated.

Abort the handshake if the `Sec-WebSocket-Protocol` header is invalid.

Make the parser throw an error if the header field value is empty or if
it begins or ends with a white space.

Make the `WebSocket` constructor throw a `SyntaxError` if the URL
contains a fragment identifier or if the URL's protocol is not one of
`'ws:'`, `'wss:'`, or `'ws+unix:'`.

Avoid decoding text messages and close reasons to strings. Pass them as
`Buffer`s to the listeners of their respective events. Also, make
listeners of the `'message'` event take a boolean argument to speficy
whether or not the message is binary.

Refs: #1878
Refs: #1804

Fixes #1886

When `WebSocketServer.prototype.close()` is called, stop accepting new
connections but do not close the existing ones.

If the HTTP/S server was created internally, then close it and emit the
`'close'` event when it closes. Otherwise, if client tracking is
enabled, then emit the `'close'` event when the number of connections
goes down to zero. Otherwise, emit it in the next tick.

Refs: #1902

Match the behavior of Node.js core `net.Server` and call the callback
of `WebSocketServer.prototype.close()` with an error if the server is
already closed.

Refs: ecb9d9ea

Make `WebSocket.prototype.addEventListener()` a noop if the `type`
argument is not one of `'close'`, `'error'`, `'message'`, or `'open'`.

Make `WebSocket.prototype.removeEventListener()` only remove listeners
added with `WebSocket.prototype.addEventListener()` and only one at
time.

Prevent the `onclose`, `onerror`, `onmessage`, and `onopen` getters and
setters from returning or removing event listeners added with
`WebSocket.prototype.addEventListener()`.

Also prevent `WebSocket.prototype.removeEventListener()` from removing
event listeners added with the `onclose`, `onerror`, `onmessage`, and
`onopen` setters.

Refs: #1818

Make the `onclose`, `onerror`, `onmessage`, and `onopen` getters return
`null` instead of `undefined` if the event handler is not set.

- Remove non-standard `OpenEvent` class.
- Make properties read-only.
- Update constructor signatures to match the ones defined by the HTML
  standard.