Add a general way to request an immediate top-level callback.

This is a little like schedule_timer, in that the callback you provide
will be run from the top-level message loop of whatever application
you're in; but unlike the timer mechanism, it will happen
_immediately_.

The aim is to provide a general way to avoid re-entrance of code, in
cases where just _doing_ the thing you want done is liable to trigger
a confusing recursive call to the function in which you came to the
decision to do it; instead, you just request a top-level callback at
the message loop's earliest convenience, and do it then.

[originally from svn r10019]

windows/window.c

@@ -869,6 +869,8 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 	} else
 	    sfree(handles);
 
+        run_toplevel_callbacks();
+
 	while (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {
 	    if (msg.message == WM_QUIT)
 		goto finished;	       /* two-level break */
@@ -877,12 +879,11 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 		DispatchMessage(&msg);
 	    /* Send the paste buffer if there's anything to send */
 	    term_paste(term);
-	    /* If there's nothing new in the queue then we can do everything
-	     * we've delayed, reading the socket, writing, and repainting
-	     * the window.
-	     */
+
 	    if (must_close_session)
 		close_session();
+
+            run_toplevel_callbacks();
 	}
 
 	/* The messages seem unreliable; especially if we're being tricky */

windows/winplink.c

@@ -738,6 +738,8 @@ int main(int argc, char **argv)
 	    }
 	}
 
+        run_toplevel_callbacks();
+
 	if (n == WAIT_TIMEOUT) {
 	    now = next;
 	} else {

windows/winsftp.c

@@ -585,6 +585,8 @@ int do_eventsel_loop(HANDLE other_event)
 
     sfree(handles);
 
+    run_toplevel_callbacks();
+
     if (n == WAIT_TIMEOUT) {
 	now = next;
     } else {