iwd: Register the Netconfig agent

Handle IP Configuration requests from IWD so that, when IWD's main.conf
setting [General].NetworkConfigurationEnabled is true, we don't try to
run DHCP or static addressing in parallel with IWD's internal DHCP or
static addressing.

Since part of the IWD secret agent and the new NetConfig agent
registration code is common, the agent object's path is changed.