(ps-mode-print-function): Fix default

(ps-mode-print-function): Fix default
value: \"lpr\" changed to "lpr"
(ps-mode-version): New constant
(ps-mode-show-version): New function, added key in ps-mode-map
(ps-run-messages): Removed
(ps-run-font-lock-keywords-2): New defcustom variable replacing
ps-run-messages. These keywords now include the value of
ps-run-prompt, making its fontification customizable.
(ps-run-init): Removed \\n from docstring, it is now added when
the value is used
(ps-run-font-lock-keywords-1): Added checking for initial ^ in
ps-run-prompt
(ps-mode): Added ps-run-font-lock-keywords-2 to list of
customizable variables in doc-string (it's equivalent
ps-run-messages was missing in previous version of the doc-string)
(ps-run-mode): Simplified assignment to font-lock-defaults, using
symbols only