Added handle_t type to server interface so that we can make handles
pointers later on.
Always use 0 to signal invalid handle in server requests.

diff --git a/server/queue.c b/server/queue.c
index 8d63e7b..3067c0c 100644
--- a/server/queue.c
+++ b/server/queue.c
@@ -110,7 +110,7 @@
 {
     struct msg_queue *queue = current->queue;
 
-    req->handle = -1;
+    req->handle = 0;
     if (!queue) queue = create_msg_queue( current );
     if (queue) req->handle = alloc_handle( current->process, queue, SYNCHRONIZE, 0 );
 }