Implement an auth_hint for the standalone plugin

[alexzorin]

It looks like we forgot to do that, like we did for all the other built-in plugins. Right now it shows the default auth_hint, but we can do better.

I did a quick survey of standalone-related threads on the community forums and these are the problems that are cropping up:

Forum Thread	What's the problem?
Thread	Choice of plugin: Running inside a k8s pod, certbot --standalone probably not suitable.
Thread	Choice of plugin: The server was an Alibaba Object Bucket, certbot --standalone was not suitable.
Thread	DNS issue: Domain was pointing to a domain masking service, breaking HTTP DCV.
Thread	DNS issue: Wrong DNS records.
Thread	DNS issue: Wrong IPv6 address.
Thread	DNS issue: Wrong IPv6 addresses.
Thread	Port forwarding: traffic going to wrong server.
Thread	Port forwarding: traffic going to wrong server.
Thread	Proxy issue: haproxy was doing health checks and it wasn't seeing the Certbot standalone server come up quickly enough.
Thread	Proxy issue: web host intercepting /.well-known/acme-challenge requests.
Thread	Unclear. Either nginx is already running, or Certbot is running on the wrong machine.
Thread	Unclear. Either wrong IP address or some exotic port forwarding.
Thread	Unclear. Either wrong IP address or some exotic port forwarding.
Thread	Unclear. Either wrong IP address or some exotic port forwarding.
Thread	Unclear. Possibly proxy problem with AWS ELB, or locally running nginx server.
Thread	Unclear. Tomcat was probably still running.

The main things seem to be:

1. Form a useful mental model. Convey that Certbot is starting up a temporary web server, and that the Certificate Authority is trying to download some challenge files from it via the user's domain name.
2. Get the user to confirm that the domain name is pointing to the machine where Certbot is actually running. That covers perhaps upto 75% of the issues listed above. I don't know how many of the others we should mention.
3. Potentially, change the language of StandaloneBindError to be like, "yo, something is definitely already running on http01_port. You need to make sure it's stopped, see [...]".

[alexzorin]

Potentially, change the language of StandaloneBindError to be like, "yo, something is definitely already running on http01_port. You need to make sure it's stopped, see [...]".

Turns out the standalone plugin is literally meant to do this, but the functionality was lost during a refactor of acme.standalone.