Can't just check whether we have existing system secrets, because
that doesn't catch the case for a completely new connection where
there may not be any secrets yet, but any that we do get should
be system-owned.
When a connection is visible only to one user, check 'own' instead
of 'system', allowing 'own' to be less restrictive since the change
won't affect any other users.
Meaning stays the same, but this will allow us to differentiate
in the future between personal connections (ie, just visible to
one user) and system connections (visible to more than one user).
We need to iterate through each item in the VPN's 'secrets' property
and mark it as not required, instead of just marking the 'secrets'
property itself as not required. Yeah, VPN secrets are a bit
annoying.
Do the check for system-owned secrets once, before kicking off the
request, instead of each time we ask an agent. As a bonus, this
change ensures priv->secrets doesn't store anything except
system-owned secrets too, simplifying some checks later on.
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.
