doc: Improve setup for documentation building with FOP

Add a makefile rule for building PDFs with FOP.  Two new build targets
in doc/src/sgml are postgres-A4-fop.pdf and postgres-US-fop.pdf.

Run .fo output through xmllint for reformatting, so that errors are
easier to find.  (The default output has hardly any line breaks, so you
might be looking for an error in column 20000.)

Set some XSLT parameters to optimize for building with FOP.

Remove some redundant or somewhat useless chapterinfo/author
information, because it renders strangely with the FO stylesheet.

Reviewed-by: Álvaro Herrera <alvherre@2ndquadrant.com>