budget: Track sector expenses in nat_budget[]

budget: Track sector expenses in nat_budget[]

The update summarizes sector production, building and maintenance for
budget in a two-dimensional array int p_sect[SCT_BUDG_MAX+1][2].  All
references into this array use literals as second subscript.  Bzzzt,
wrong data type.

Add two one-dimensional arrays to nat_budget[], one for production,
and one for building and maintenance.  p_sect[i] becomes
nat_budget[cnum].prod[i] for production, and .bm[j] for building and
maintenance.  p_sect[i][0] becomes .count, and -p_sect[i][1] becomes
.money.

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