DEFENSE_INFRA was implemented in an odd way: sct_defense was used

DEFENSE_INFRA was implemented in an odd way: sct_defense was used
regardless of the option, but forced to sct_effic when disabled.  This
screws up sct_defense when you disable DEFENSE_INFRA.  Implement it
more like FALLOUT: use sct_defense if enabled, else sct_effic.  The
change should be invisible except in xdump, which shows the real
sct_defense.  Closes #804641.
(SCT_DEFENSE): New.
(dump, sinfra, sector_strength): Use it.
(eff_bomb, build_bridge, build_tower, new, buildeff, sect_damage)
(put_combat, checksect, produce_sect): Don't force sct_defense to
sct_effic when DEFENSE_INFRA is disabled.