(\onepageout): \ifcropmarks, center the page body.

Lost this when merged \croppageout with \onepageout.
Found by Arnold.

(\doprintindex): Do not bother to go into double column mode unless
there actually is a non-empty index.

(\begindoublecolumns): Include any existing \partialpage in the new one,
lest we lose a whole page of output.
Found by M J Morley <mjm@scs.leeds.ac.uk>.

(\chapternofonts): Remove spurious spaces, both in the definitions that
get output to the aux file(s) and in this macro.

Fix comments and rationalize whitespace in various other places.

From-SVN: r14015