move parse_xrefs back next to main

Move parse_xrefs() back in its proper place as a helper to main(),
including the definition of the global variable 'infile' with the rest
of the command line arguments.

It had been moved in fbbe953f when the content processing code was
confusedly hooked into the function.

Also removes marker comments about "Start/End xref parsing". The code
between them is not exclusively concerned with xrefs and their sheer
size clashes with the rest of the coding style.