Unix domain socket support

[NOTE]: # ( ^^ Provide a general summary of the request in the title above. ^^ )

## Summary

[NOTE]: # ( Provide a brief overview of what the new feature is all about. )

I want to run CouchDB behind a reverse proxy by listening to the unix domain socket.

## Desired Behaviour

[NOTE]: # ( Tell us how the new feature should work. Be specific. )
[TIP]: # ( Do NOT give us access or passwords to your actual CouchDB! )

```ini
[chttpd]
bind_address = /path/to/couchdb.sock
```

or

```ini
[chttpd]
socket_path = /path/to/couchdb.sock
```

Then CouchDB will accept requests from `/path/to/couchdb.sock`.

## Possible Solution

[NOTE]: # ( Not required. Suggest how to implement the addition or change. )

## Additional context

[TIP]: # ( Why does this feature matter to you? What unique circumstances do you have? )

This is because I want to run CouchDB on a shared host where I can't have exclusive access to any ports.

No source files or tests are named. Start by tracing the [chttpd] bind_address configuration and the HTTP listener entry point, then compare the proposed bind_address and socket_path forms. Done means CouchDB accepts requests through the configured Unix domain socket and preserves the existing port-listening behavior.

Written by the indexing model from the issue text.