act-request: allow omitting the @self argument in nm_act_request_cancel_secrets()

Previously, we would require a @self argument and the @call_id in
nm_act_request_cancel_secrets(), although the @call_id already has
a pointer to @self.
In principle that is not necessary, but it makes the API a bit
more robust as you need to care about the lifetime of the @req
as well.

However it is a bit inconvenient, because it requires that caller to
track both the activation request and the call-id.

Now, allow nm_act_request_get_secrets() to instruct the call-id to
take an additional reference to @self. Later on, we would allow to omit
the argument during cancelling. We only allow this, if the call-id
takes a reference to @self.

src/nm-act-request.h

@@ -63,6 +63,7 @@ typedef void (*NMActRequestSecretsFunc) (NMActRequest *req,
                                          gpointer user_data);
 
 NMActRequestGetSecretsCallId nm_act_request_get_secrets (NMActRequest *req,
+                                                         gboolean take_ref,
                                                          const char *setting_name,
                                                          NMSecretAgentGetSecretsFlags flags,
                                                          const char *hint,