Universal Port: Simplified administration for rippled servers

This implements part of the "Universal Port" feature. It allows peer, RPC over http, RPC over HTTPS, Websocket, and Secure Websockets to be served on a single listening port. In this version, websockets is disabled and only legacy peer connections are allowed to become peers. Peer HTTP upgrade requests are given a "503 Service Unavailable" with a JSON content body with a handful of IP addresses to visit.
Reviewers: @ximinez, @JoelKatz 
In terms of attack surface area, a random IP address with access to your admin controls is miniscule, but it's not zero. Can we suggest using a non-routable IP instead?

```
# If you want to test the output of non-admin commands add this section and
# just put a private IP address that does not exist on your local network
# (eg. 192.168.255.254, 10.0.0.0, or 168.254.15.20).
```