xmpp_destroy_buddy() now accepts an 'aggressively' flag that indicates whether the...

xmpp_destroy_buddy() now accepts an 'aggressively' flag that indicates whether the server should push an 'unsubscribed' presence tag.  Apparently we can only do this when sending a roster flush at the beginning of the session, because sending it when another user logs out in the middle of our session confuses Jitsi into thinking that the other user rejected a subscription request.  I think it's making Pidgin behave better too, but more testing is needed.