Make sector maintenance cost configurable

Make sector maintenance cost configurable

Replace the fixed $1 per ETU maintenance for capital/city sectors that
are at least 60% efficient by a configurable maintenance cost, payable
regardless of efficiency.  The only change in the default
configuration is that inefficient capitals now pay maintenance.
Charging sector maintenance regardless of efficiency is consistent
with unit maintenance.

New struct dchrstr member d_maint and sector-chr selector maint.  Make
show_sect_build() show it.  Change produce_sect() to record
maintenance in new slot p_sect[SCT_MAINT] instead of abusing
p_sect[SCT_CAPIT].  Replace the "Capital maintenance" line in budget
by "Sector maintenance".