core: implement connection assumption

Mark activation requests that contain connections to be assumed,
and use that to short-circuit various parts of the activation
process by not touching various device attributes, since they
are already set up.  Also ensure the device is not deactivated
when it initially becomes managed, because that would kill the
connection we are about to assume.

src/nm-activation-request.h

@@ -59,6 +59,7 @@ GType nm_act_request_get_type (void);
 NMActRequest *nm_act_request_new          (NMConnection *connection,
                                            const char *specific_object,
                                            gboolean user_requested,
+                                           gboolean assumed,
                                            gpointer *device);  /* An NMDevice */
 
 NMConnection *nm_act_request_get_connection     (NMActRequest *req);
@@ -85,6 +86,8 @@ void          nm_act_request_add_share_rule (NMActRequest *req,
 
 GObject *     nm_act_request_get_device (NMActRequest *req);
 
+gboolean      nm_act_request_get_assumed (NMActRequest *req);
+
 gboolean nm_act_request_get_secrets    (NMActRequest *req,
                                         const char *setting_name,
                                         gboolean request_new,