Creating a key binding which replaces itself (such as "bind x bind x lsw")

frees the command list bound to the key while it is still being executed,
leading to a use after free. To prevent this, create a dead keys list and defer
freeing replaced or removed key bindings until the main loop when the key
binding will have finished executing.

Found by Johan Friis when creating a key binding to reload his configuration
file.

server.c

@@ -346,6 +346,9 @@ server_main(int srv_fd)
 		server_handle_windows(&pfd);
 		server_handle_clients(&pfd);
 
+		/* Collect any unset key bindings. */
+		key_bindings_clean();
+		
 		/*
 		 * If we have no sessions and clients left, let's get out
 		 * of here...