There are three additions to the D-Bus interface for VPN plugins as
part of this patch:

1) ConnectInteractive(): called by NM instead of Connect() to let
the plugin know that it can request additional secrets during the
connection process using SecretsRequired

2) SecretsRequired: a new signal emitted by the plugin to indicate
to NetworkManager that additional secrets are required to connect;
can only be called if NetworkManager initiated the connection by
calling the ConnectInteractive() method

3) NewSecrets(): a new method of the plugin that NetworkManager calls
when new secrets requested by the SecretsRequired signal have been
retrieved from secret agents

We need new methods because agents need to be aware of the hints that
the VPN plugins may send with the SecretsRequired signal (detailing
the specific secrets that are required) and at this time, not all
agents support passing those hints to the VPN plugin authentication
dialogs.