process page content after main parser and only for text extraction

Factors this code out of parse_xrefs() where it never belonged, into a
new function process_page_content() that is called from main after the
main parse has succeeded and only if text extraction was requested, i.e.
-x or -X was passed on the command line.

Also adjusts the code for style and drops some related XXXs.

Fixes #49.