config: Make work to build sectors configurable

config: Make work to build sectors configurable

Traditionally, building up 100% takes 100 work.  Make the work to
build configurable, via new sect-chr selector bwork, backed by new
struct dchrstr member d_bwork.  Keep the required work exactly the
same for now.

Tearing down sectors remains four times easier than building.

Clients that hardcode sector build work need to be updated.  Easy,
since build work is now exposed in xdump.

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