* lily/page-turn-page-breaking.cc (calc_demerits): now that we allow

	* lily/page-turn-page-breaking.cc (calc_demerits): now that we allow
	put_systems_on_pages to return an empty result, we need to assign
	demerits properly in that case.
	(put_systems_on_pages): Make the number of pages depend on the
	evenness of page_number. Add auto-first-page-number.
	(calc_subproblem): ensure that the page-number is even for the
	left-hand page. Warn if the first page-turn doesn't fit onto the
	first (right-hand) page.

	* lily/page-spacing.cc (solve): bug: demerits_ would always be inf
	(min_page_count): make this publicly accessible
	(min_page_count): fix a bug when there are forced page breaks that
	was introduced when I made this loop run backwards
	(space_systems_on_n_pages, space_systems_on_n_or_one_more_pages):
	replace space_systems_on_min_pages with these two. The logic in
	space_systems_on_min_pages was getting too convoluted and is better
	contained in page-turn-page-breaking.