Make the witness lock limit an option.
Add required `exec' before Xdialog(1) invocation for the case where we
expect (require) data on stdin. The effect of this is that, when
backgrounded, $! returns the pid of an Xdialog(1) instance instead of
an sh(1) instance -- the difference being a later kill of $! will reach
the Xdialog(1) instance whereas without the exec the kill succeeds on
the sh(1) instance leaving Xdialog(1) open.