Fix bug 1: if ps-font-size-internal,

Fix bug 1: if ps-font-size-internal,
ps-header-font-size-internal and
ps-header-title-font-size-internal variables are not set,
ps-nb-pages and ps-line-lengths-internal crashes.  Fix bug 2: if
face text property is (foreground-color . COLOR) or
`(background-color . COLOR)', ps-print crashes.  Doc fix.
(ps-print-version): New version number (5.2.4).
(ps-plot-region): Code fix.
(ps-nb-pages, ps-line-lengths-internal): Bug fix 1.
(ps-face-attribute-list, ps-face-attributes, ps-face-background):
Bug fix 2.