config: Make product work independently configurable

The work required for a product is traditionally the amount of raw
materials, plus 1 for resource usage, or 1 if using neither.  Make it
independently configurable instead, via new product selector bwork,
backed by new struct pchrstr member p_bwork.  Keep the required work
exactly the same in the default configuration.

Clients that compute work from materials need to be updated.  Easy,
since build work is now exposed in xdump.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>