octave --force-gui no longer forces the GUI

[Mike Miller]

Rik,

Something in the logic switch in changeset 25b6fbe82827 has made the
--force-gui option not work anymore if stdin is not a terminal. So for
example

  $ octave --force-gui

works, but

  $ octave --force-gui < /dev/null

does not, it just returns to the shell. This breaks running the GUI
from a launcher or from the desktop environment shell (e.g. Alt-F2 in
gnome).

It's not obvious to me what's wrong in the diff, but it works
correctly on the previous revision.

-- 
mike