VPN secrets are currently unhandled by the UI code. To avoid
lengthy timeouts, bail out early with an error, so NetworkManager
falls back to the nm-applet agent directly.
https://bugzilla.gnome.org/show_bug.cgi?id=658484
A network agent is a component that stores network secrets (like
wifi passwords) in the session keyring. This commit adds an
implementation of it to be used by the shell network dialogs. It
handles most of the keyring stuff, delegating the UI to upper layers.
https://bugzilla.gnome.org/show_bug.cgi?id=650244