Change the policy of background/foreground default color.

Change the policy of background/foreground default color.
(ps-print-version): New version number (6.5.9).
(ps-default-fg, ps-default-bg): Adjust default value.
(ps-begin-file, ps-begin-job): ps-default-fg and ps-default-bg variables now use
`t' to indicate the use of Emacs session background/foreground color.
(ps-output-string-prim): If index out of range, insert hexadecimal
representation of character.