There are now two major axes for a backend config, and the following
combinations are allowed.
* "proto" is the protocol:
* AFSK
* APRS-IS
* KISS
* Kenwood
* "link" is the connection type for KISS and Kenwood:
* USB Serial
* Bluetooth SPP
* TCP/IP (client)
* "aprsis" is the connection type for APRS-IS:
* TCP connection
* HTTP POST
* UDP (send only)
"backend" is the effective configuration that results from "proto" and
"link"/"aprsis", and it's responsible for creating the right backend
class. The backend class will then use the appropriate protocol handler.