Let players stop/start units:

Let players stop/start units:
(genitem, lndstr, nukstr, plnstr, shpstr): New members off, lnd_off,
nuk_off, pln_off, shp_off.
(NSC_GENITEM): New selector off.
(land, nuke, plan, shi): Display efficiency prefixed by `=' when off.
(start, stop, player_coms): Rewrite, new syntax.  Print only objects
that actually change.
(start_hdr, stop_hdr, start_stop_hdr): Consolidate.
(item_u, start_stop, start_stop_sector, proff, start_stop_unit)
(start_stop_unit_hdr, unit_type_name): New.
(upd_land, upd_plane, planerepair, upd_ship): Obey and clear stoppage.