auth: move common nm_dbus_manager_get_caller_info() functionality into nm_auth_chain_new()

Most callers of nm_auth_chain_new() call nm_dbus_manager_get_caller_info()
right before that, so just fold the get_caller_info() call into
nm_auth_chain_new() to reduce code complexity in callers.  Yes, this
means sometimes we call nm_dbus_manager_get_caller_info() twice,
but that's not really a problem.