Issue 4912: Fix output definition use in \book and \bookpart

The only explicit output definition blocks allowed in \book and
\bookpart blocks were paper blocks.  Output definitions supplied with
Scheme expressions were erroneously interpreted like global output
definitions, accepting all output definition types and overriding the
global defaults with them.

Now the only output definitions accepted as Scheme expressions are
paper blocks.  As opposed to previously, they actually set the paper
block of the respective book or bookpart.