tools/books*py: improve argument handling of DOC_LANG, and others

1. fix shebang lines;
2. add a -o option that makes no changes but prints the files that would be updated/created;
3. allow the -l argument (and therefore DOC_LANG) to be whitespace separated, comma separated, or a mixture of both

Item 2 allows us to start tracking dependencies in Makefiles if we wish
and therefore reintroduce incremental builds, reducing the amount of
.PHONY targets that are not truly phony.

Item 3 restores compability with the documented DOC_LANG variable, which
was previously whitespace separated.

Reviewed by: dbaio, blackend
Differential Revision: https://reviews.freebsd.org/D30732