IP encapsulation spec is reflector specific

[nivex]

The IP Networking section (docs/ip_encapsulation.rst) of the specification refers specifically to reflectors throughout. While this is currently the only method of doing M17 Internet linking, a healthy discussion happened on chat today that brought about the notion of a system that could route packets based on the encapsulated packet's destination field.

I therefore propose:

• all references to "reflector" in this specification be changed to the more generic "server"
• the module parameter (byte 10) to the CONN command
  • be made optional
  • be further explained that its purpose is for reflector use and a link to the reflector documentation be included

If we are not amenable to having the module parameter be optional, I propose an additional character '+' be allowed to indicate the client expects destination field routing instead of reflector style functionality.