_nm_connection_for_each_secret() (formerly for_each_secret()) and
_nm_connection_find_secret() (formerly find_secret()) operate on a
GVariant of secrets. For that, they implement certain assumptions
of how to handle secrets. For example, it must special-case VPN settings,
because there is no generic abstraction to handle regular secret and VPN
secrets the same.

Such special casing should only be done in libnm-core, at one place.

Move the code to libnm-core as internal API.