update: Saner rounding of sector building money and work

update: Saner rounding of sector building money and work

buildeff() rounds work and money up.  Until recently, fractions could
only occur on tear-down, but with customized costs they can now also
occur on build-up.

The previous commit changed unit building to round money and work
randomly.  Before, money was rounded down, and work was rounded up.

Round them randomly for sectors as well, for consistency.

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