CR-6519 OPENDJ-1899 Generate multiple man pages for dsconfig

This patch splits the dsconfig man page into a dsconfig page
that lists the subcommands, and a page per subcommand.

In the Reference the subcommands pages are included
in their own reference section,
because there's no good way to present the whole list
in the midst of the tools reference.

As a side effect this patch makes it possible to build PDF again
(and to build man pages) without running out of memory.