networkAgent: pass VPN hints to auth dialogs

Indicate to NetworkManager that the Shell's agent supports VPN
hints, and pass those hints to VPN auth dialogs that also indicate
that they support hints.

VPN plugins can request new secrets, for example if the previous
ones are incorrect (eg, user mis-typed the password) or some other
reason (next token code required to re-sync a hardware token).
The specific secret that the VPN wants, and a VPN-specific message,
are passed in hints from the plugin, to NetworkManager, to the
agent (GNOME Shell) and then to the auth dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=737592