output filename of PostScript image

[Qian Yun]

Currently we have image.bm, image.xpm for bitmap image,
but for PS image, we have fricas2D.ps and fricas3D.ps.

I suggest to use "image.ps" instead, opinions?

- Qian

[Ralf Hemmecke]

Interestingly, in ug07.htex there also appears fricas3D.pixmap.
Also in

./src/graph/view3D/process3d.c:0743:

I see fricas3D.xpm mentioned. Change that as well?

I do not think it would be much of a problem if the generated .ps files
are called image.ps. In fact, I don't care much about the name, but
maybe in view of a common name I would prefer image.ps.

Ralf