* ps-print.el: Compatibility, customization and doc fix.

* ps-print.el: Compatibility, customization and doc fix.
(ps-printer-name-option): Replace defconst by defvar.
(ps-postscript-code-directory): XEmacs compatibility.
(ps-header-sheet, ps-setup, ps-begin-file, ps-begin-job): Code
fix.
(ps-user-defined-prologue, ps-print-prologue-header)
(ps-xemacs-face-kind-p, ps-face-bold-p, ps-face-italic-p): XEmacs
compatibility and code fix.
(ps-print-background-image, ps-print-background-text):
Customization fix.
(ps-line-number-start, ps-n-up-on): New vars.