dispatcher-connection: add new 'disconnect-request' event

When bearer gets an event that should trigger a disconnection but we
cannot disconnect (for example because we're waiting for pppd to close),
we now trigger a new 'disconnect-request' event instead

This was tested with the following hook:
```
$ cat /etc/ModemManager/connection.d/handle-disconnect-request.sh
#!/bin/sh

if [ "$4" != disconnect-request ]; then
	exit 0
fi

iface=$(mmcli -b "$2" -K | awk '/bearer.status.interface/ { print $3 }')

if pkill -f "pppd\>.*\<$iface\>"; then
	logger -t mm-disconnect-request "Killed $iface pppd"
else
	logger -t mm-disconnect-request "No pppd found for $iface"
fi
```

Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>

src/mm-base-bearer.c

@@ -1492,6 +1492,8 @@ mm_base_bearer_disconnect_force (MMBaseBearer *self)
 
     if (self->priv->ignore_disconnection_reports) {
         mm_obj_dbg (self, "disconnection should be forced but it's explicitly ignored");
+        bearer_run_dispatcher_scripts (self,
+                                       MM_DISPATCHER_CONNECTION_EVENT_DISCONNECT_REQUEST);
         return;
     }
 

src/mm-dispatcher-connection.c

@@ -66,6 +66,8 @@ mm_dispatcher_connection_event_to_string (MMDispatcherConnectionEvent event)
         return g_strdup ("connected");
     case MM_DISPATCHER_CONNECTION_EVENT_DISCONNECTED:
         return g_strdup ("disconnected");
+    case MM_DISPATCHER_CONNECTION_EVENT_DISCONNECT_REQUEST:
+        return g_strdup ("disconnect-request");
     default:
         return NULL;
     }

src/mm-dispatcher-connection.h

@@ -35,6 +35,7 @@ typedef struct _MMDispatcherConnectionPrivate MMDispatcherConnectionPrivate;
 typedef enum { /*< underscore_name=mm_dispatcher_connection_event >*/
     MM_DISPATCHER_CONNECTION_EVENT_CONNECTED,
     MM_DISPATCHER_CONNECTION_EVENT_DISCONNECTED,
+    MM_DISPATCHER_CONNECTION_EVENT_DISCONNECT_REQUEST,
 } MMDispatcherConnectionEvent;
 
 GType                   mm_dispatcher_connection_get_type   (void);