If all agents can handle VPN hints, then we'll try to use
ConnectInteractive() to let the VPN plugin ask for secrets
interactively via the SecretsRequired signal.  These hints
are then passed to agents during the connection process if
the plugin needs more secrets or different secrets, and when
the new secrets are returned, they are passed back to the VPN
plugin.

If at least one agent does not have the VPN hints capability,
we can't use ConnectInteractive(), but fall back to the old
Connect call, because that agent won't be able to send the
hints to the VPN plugin's authentication dialog, and thus
we won't get back the secrets the VPN plugin is looking for.

So, for interactive secrets to work correctly, you need:

1) A VPN plugin updated for interactive secrets requests
2) NM updated for interactive secrets requests
3) all agents to set the VPN_HINTS capability when
    registering with NetworkManager and to pass hints
    along to the VPN authentication dialog
4) a VPN authentication dialog updated to look for hints
    and only return secrets corresponding to the hints
    requested by the plugin