ps-default-bg shouldn't be used for printing by default

[Christian Schlauer]

,----[ C-h v ps-default-bg RET ]
| ps-default-bg is a variable defined in `ps-print.el'.
| Its value is frame-parameter

[...]

| The `ps-default-bg' variable contains the default background color
| used by ps-print, that is, if there is a face in a text that doesn't
| have a background color, the `ps-default-bg' color should be used.

[...]

| It's used only when `ps-print-color-p' is non-nil.

[...]

`----

`ps-print-color-p' is t by default -- that means that the background
colour of my frame will be printed when I use M-x
ps-print-buffer-with-faces.

The problem is that ink jet printers today are mostly colour ink jet
printers, and even if I want the faces of a buffer to appear in colour
on paper, I am not interested in the background colour of the frame.
It is a waste of ink and _time_ as it takes much longer to print the
pages (the same holds for colour laser printers: a waste of toner and
_time_).

My proposal: change the default of `ps-default-bg' to white colour or
some equivalent expression.

FWIW, the Firefox web browser doesn't print the background colours or
background images of web pages by default: have a look in File -> Page
Setup... -> Format & Options: the "Print background (colours &
images)" option is _not_ checked by default. That's very sensible.
This would make sense for Emacs, too -- I guess many people choose
another background colour for their Emacs frames than white. I use
`ivory', for example.