Set the global variable eval_exit.

  This is set if *both* --eval and --exit are specified on the
 command line.  Because this implies that our curses UI won't
 be visible - or at least not for long.
  If eval_exit is present then write output to stdout.

  This is helpful to allow:

        lumail --eval "daily();" --exit

  The output from msg() will now go to the invoking console, rather
 than being lost.