It's the thing that owns the secrets anyway, and it simplifies things to
have the secrets handling there instead of half in NMActRequest and
half in NMManager. It also means we can get rid of the ugly signals
that NMSettingsConnection had to emit to get agent's secrets, and
we can consolidate the requests for the persistent secrets that the
NMSettingsConnection owned into NMSettingsConnection itself instead
of also in NMAgentManager.
Since the NMActRequest and the NMVPNConnection classes already tracked
the underlying NMSettingsConnection representing the activation, its
trivial to just have them ask the NMSettingsConnection for secrets
instead of talking to the NMAgentManager. Thus, only the
NMSettingsConnection now has to know about the agent manager, and it
presents a cleaner interface to other objects further up the chain,
instead of having bits of the secrets request splattered around the
activation request, the VPN connection, the NMManager, etc.
Allows NM to signal to an agent that an in-progress secrets
request is no longer needed. This could happen if the device
for which the request was started was removed, disabled, or the
network being connected to changed, or whatever.
Filter registered agents for each secrets request to ensure that the
connection for which secrets are requested is visible to that agent,
and add that agent to the queue. Ask each agent in the queue until
one returns usable secrets. Ensure that if new agents register
or existing agents quit during the secrets request, that the queue
is updated accordingly, and ensure that an agent that's already
been asked for secrets, unregisters, and re-registers before the
secrets request is comple, isn't asked for secrets twice.
