Add further missing delete_callbacks_for_context.

Having explicitly _stated_ in commit 4dcc0fddf the principle that if
you ever queue a toplevel callback on a freeable object then you
should also call delete_callbacks_for_context on that object before
freeing it, I realised I'd never actually gone through and checked
methodically at every call site of queue_toplevel_callback. So I did,
and naturally, I found several missing ones.

scpserver.c

@@ -521,6 +521,9 @@ static void scp_source_free(ScpServer *s)
         scp->head = node->next;
         sfree(node);
     }
+
+    delete_callbacks_for_context(scp);
+
     sfree(scp);
 }
 

terminal.c

@@ -1804,6 +1804,7 @@ void term_free(Terminal *term)
     sfree(term->tabs);
 
     expire_timer_context(term);
+    delete_callbacks_for_context(term);
 
     conf_free(term->conf);
 

windows/winhsock.c

@@ -128,6 +128,8 @@ static void sk_handle_close(Socket *s)
         CloseHandle(hs->recv_H);
     bufchain_clear(&hs->inputdata);
 
+    delete_callbacks_for_context(hs);
+
     sfree(hs);
 }
 

windows/winnet.c

@@ -1344,6 +1344,7 @@ static void sk_net_close(Socket *sock)
     p_closesocket(s->s);
     if (s->addr)
 	sk_addr_free(s->addr);
+    delete_callbacks_for_context(s);
     sfree(s);
 }