fb_friend: problems for multiple call of FB_FRIEND_OP_REQUEST_ACCEPT

Hi,
assumed, that a FB user is inveted by many users for the same app, i think the input for trigger FB_FRIEND_OP_REQUEST_ACCEPT needs to be improved.

if the invited user accepts all the app-invitations and fb_friend_request_accept_page() is called each time, FB_FRIEND_OP_REQUEST_ACCEPT would be always called with all the inviter as data input.

If you give the inviter userpoints or any other credit, they would be very happy, to get it multiple times.

The problem is, that it is ignored, that the invite status is set to FB_FRIEND_STATUS_REQUEST_ACCEPTED after the first call of the function.

I will add a patch who fixes this.