Refactor player-set-target SEXP

[LuytenFS]

Based on prior feedback from @Goober5000, I've revisited this feature to address the following:

1. the sexp implementation uses ship_name_lookup, but eval_ship would take advantage of the ship registry (faster lookup that comes with handling of non-present states) and would also cache the ship in the sexp node
2. last_targeted_subobject shouldn't be used; the sexp implementation should explicitly set the subsystem (as it does in the clearing sexp) without using hacks

However I should address the line:

    // set_target_objnum will call hud_restore_subsystem_target which reads last_targeted_subobject
    shipp->last_targeted_subobject[Player_num] = new_subsys;

HUD code still reads ship::last_targeted_subobject instead of Player_ai->targeted_subsys I think am not sure, so SEXPs have to write it as a hack. Needs HUD‑side fix so hud_restore_subsystem_target uses targeted_subsys; then last_targeted_subobject can be removed from SEXP logic. I'd have to say that this is an assumption.