Prevent access to Fauxton on node-local port (5986)

[wohali]

Will help stop people shooting themselves in the foot and/or using
node-local CouchDB as their "main" CouchDB port.

I'll file a separate docs repo PR on this once this merges.

Test results:

$ curl -v http://localhost:15986/_utils
*   Trying ::1...
* TCP_NODELAY set
* connect to ::1 port 15986 failed: Connection refused
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 15986 (#0)
> GET /_utils HTTP/1.1
> Host: localhost:15986
> User-Agent: curl/7.52.1
> Accept: */*
>
< HTTP/1.1 410 Gone
< Cache-Control: must-revalidate
< Content-Length: 110
< Content-Type: application/json
< Date: Sat, 03 Mar 2018 00:45:22 GMT
< Server: CouchDB/2.2.0-51cb6aecc (Erlang OTP/19)
<
{"error":"no_node_local_fauxton","reason":"The web interface is no longer available on the node-local port."}

Closes #1198

[chewbranca]

This should be indented one space further in, eg 4 spaces instead of 3.

[chewbranca]

+0.5 from me (after the WS nit). The approach seems fine, but I don't have strong enough opinions on removal of Fauxton from :5986 to give it a full +1, so it would be good to get some more feedback.

[janl]

I like this. How about printing a link to the cluster port URL? So users who make the mistake are a click away from where they want to be. I'd not suggest to redirect automatically, though, so users see what they got "wrong".

[rnewson]

losing the CSP tests seems a bit unfortunate.

[wohali]

These were only testing CSP on the node-local port anyway. I can port them to chttpd, give me a day or two.

[wohali]

@janl Hm, if we bind chttpd to 0.0.0.0 it could be tricky to determine the right IP address to choose, 127.0.0.1 might not be visible if they are coming from outside (and mistakenly have bound httpd to 0.0.0.0 as well.) It'd sure be a nice to have, though.

[wohali]

@rnewson I've moved the CSP tests over to chttpd, where they should be anyway, and of course they pass with flying colours because we are awesome. :)

[janl]

@wohali good call, with a load balancer it gets even worse.