Make secondary explosion sound editable

As it is right now the secondary explosion sound of ships bigger than fighters and bombers is hard coded to $Name 7 in sounds.tbl. This leads to the severe limitation that that sound has to be a compromise that is suitable from transports all the way up to a juggernaut. Quoting MageKing17 from discord this variable looks like it was meant to be editable:

> "they're handled in a different file and... they're always SHIP_EXPLODE_1, it looks like
> oddly, which sound to play is stored in a variable, as though it was intended to be customizable
> somehow, but the variable is only set when defined, so there's no way to change it without code
> changes"

Please expose that variable to the ships.tbl as $SecondarySnd or $SecExpSnd or alike so that a different more fitting sound can play for each ship type.