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 
s/sending/receiving/

I messed up: my concern was over what will happen if we receive an unencrypted request on a port that is defined as SSL only. On further thought, though I realize that the way the `detector` / `create` decision is made in `do_accept` makes that impossible.