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]

unix/uxsftp.c

@@ -536,6 +536,8 @@ static int ssh_sftp_do_select(int include_stdin, int no_fds_ok)
 
     sfree(fdlist);
 
+    run_toplevel_callbacks();
+
     return FD_ISSET(0, &rset) ? 1 : 0;
 }