core: add some logging for activation and disconnection

'vperic' had an interesting problem on IRC where every 10 minutes
the ethernet would change state from ACTIVATED -> DISCONNECTED with
a reason code of 0; the only thing I can find is that something was
telling NM to activate a connection periodically, becasue that appears
to be the only place that changes state to DISCONNECTED with a
reason code of 0.  No logging; no apparent carrier changes.

So log this condition just in case we run into it later.

src/nm-manager.c

@@ -1866,6 +1866,8 @@ internal_activate_device (NMManager *manager,
 
 	/* Tear down any existing connection */
 	if (nm_device_get_act_request (device)) {
+		nm_log_info (LOGD_DEVICE, "(%s): disconnecting for new activation request.",
+		             nm_device_get_iface (device));
 		nm_device_state_changed (device,
 		                         NM_DEVICE_STATE_DISCONNECTED,
 		                         NM_DEVICE_STATE_REASON_NONE);

src/nm-policy.c

@@ -739,6 +739,8 @@ auto_activate_device (gpointer user_data)
 	if (best_connection) {
 		GError *error = NULL;
 
+		nm_log_info (LOGD_DEVICE, "Auto-activating connection '%s'.",
+		             nm_connection_get_id (best_connection));
 		if (!nm_manager_activate_connection (policy->manager,
 		                                     best_connection,
 		                                     specific_object,