Feature: daemons: Initial new server-side IPC support in attrd.

This just adds the basic framework for handling both IPC and CPG
messages in the same way. For the moment, only unknown messages are
processed.

The basic approach is that each system has its own set of callbacks
including its own dispatch function. Those dispatch functions will do
whatever is necessary to then call a common function
(attrd_handle_request) which will then call into the table of message
handlers. The initial dispatch functions should be kept minimal.