xmaxima issues on windows, but probably generic



> There are a couple of generic xmaxima problems.
> ...
> 2) Plotting doesn't work.  No ideas yet.

The default is now [PLOT_FORMAT, GNUPLOT], while for 5.9.0 it was
[PLOT_FORMAT, OPENMATH].  Setting it to OPENMATH gives me working
plotting under xmaxima.

What shall we do on windows?

PLOT_OPTIONS in 5.9.0 is

(C1) PLOT_OPTIONS;
(D1) [[x, - 3, 3], [y, - 1.75555970201398E+305, 1.75555970201398E+305], 
[t, - 3, 3], [GRID, 30, 30], [VIEW_DIRECTION, 1, 1, 1], [COLOUR_Z, FALSE], 
[TRANSFORM_XY, FALSE], [RUN_VIEWER, TRUE], [PLOT_FORMAT, OPENMATH], 
[NTICKS, 100]]

while in CVS it is

(%i4) PLOT_OPTIONS;
(%o4) [[x, - 1.75555970201398E+305, 1.75555970201398E+305], 
[y, - 1.75555970201398E+305, 1.75555970201398E+305], [t, - 3, 3], 
[GRID, 30, 30], [VIEW_DIRECTION, 1, 1, 1], [COLOUR_Z, FALSE], 
[TRANSFORM_XY, FALSE], [RUN_VIEWER, TRUE], [PLOT_FORMAT, GNUPLOT], 
[GNUPLOT_TERM, DEFAULT], [GNUPLOT_OUT_FILE, FALSE], [NTICKS, 10], 
[ADAPT_DEPTH, 10], [GNUPLOT_PM3D, FALSE], [GNUPLOT_PREAMBLE, ], 
[GNUPLOT_CURVE_TITLES, [DEFAULT]], [GNUPLOT_CURVE_STYLES, 
[with lines 3, with lines 1, with lines 2, with lines 5, with lines 4, 
with lines 6, with lines 7]], [GNUPLOT_DEFAULT_TERM_COMMAND, ], 
[GNUPLOT_DUMB_TERM_COMMAND, set term dumb 79 22], [GNUPLOT_PS_TERM_COMMAND, se#
t size 1.5, 1.5;set term postscript eps enhanced color solid 24]]