neweff production work: Fix crash for sea sector

neweff production work: Fix crash for sea sector

The work required for building sea sectors is zero in sect.config.
When a deity runs neweff or production on a sea sector, e.g. with
"neweff *", buildeff() divides by zero.  Same when a player or deity
runs work with an engineer in a sea sector.  Broken in commit
2ffd7b948 "config: Make work to build sectors configurable", v4.4.0

Fix buildeff() to avoid the division.  Change the required work to 100
in sect.config for good measure.

Cover deity use of neweff and production in tests/update.

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