update: Use a scratch sctstr for production simulation
If player->simulation, produce_sect() must not change game state,
except for sct_updated. To avoid changing sectors, it copies each
sector's sct_item[] to a scratch buffer, and tracks new designation,
efficiency and available work in local variables.
Copy the complete sector to a scratch buffer instead. This is safer,
and will permit code simplifications.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>