(nukstr): Make it a struct genitem: add members nuk_effic, nuk_mobil,

(nukstr): Make it a struct genitem: add members nuk_effic, nuk_mobil,
nuk_tech, nuk_stockpile, nuk_opx, nuk_opy, nuk_mission, nuk_radius.
These have no effect, yet.
(build_nuke): Initialize them.
(nuke_ca): Add genitem selectors: effic, mobil, tech, group, opx, opy,
mission, radius.
(tran_nuke): Zap mission, just in case somebody invents missions for
nukes.
(trade_desc): Show tech and efficiency, for consistency with other
units.