If we can authenticate the agent for 'modify' permission, then send
any existing system secrets to it as the user has permission to change
those secrets. This means the agent doesn't have to call GetSecrets()
itself, which means simpler code on the agent side for a slight LoC
hit in NM itself.
This also moves the permissions checking into the NMAgentManager to
check each agent, which is sub-optimal since now the agent manager
has to do PolicyKit stuff, but hey that's life. Agents need secrets,
and we do need to authenticate every agent before we send secrets to
them, and the NMSettingsConnection doesn't know about individual
agents at all.
If the agent returns system-owned secrets, like when activating a new
connection which was created with no secrets, make sure the agent is
authorized to modify network settings before saving or using the
new secrets.
