(nukstr): Hold one nuke instead of a stockpile: replace members nuk_n,
nuk_types by nuk_type, nuk_plane. Add nuk_land for completeness, like
nuk_ship it's not yet used.
(nuke_ca): Update accordingly: replace selectors number and types by
type and plane.
(build_nuke): Update accordingly.
(ndump, nuke): Update accordingly. Output is no longer sorted by
location, and nukes in same location no longer share id. nuke's extra
columns for nuclear plants are repeated for every nuke.
(trade_desc): Update accordingly.
(cede, grab_sect, check_nuke_ok, detonate, trade_nameof): Talk about
nukes instead of nuclear stockpiles.
(arm, disarm): Rewrite, split off new disarm(). Don't remove nuke
from the nuke file on arm. Allow usual plane syntax, not just plane
number. Change second argument from nuke type to nuke number. When
plane is already armed, ignore nuke argument and rearm (broken in
4.2.6).
(tran_nuke): Update for changed struct nukstr, make as similar as
possible to tran_plane. Change syntax to match transport plane.
(player_coms): Update arm, disarm and transport accordingly.
(prnuke, nuk_on_plane): New.
(nuk_add, nuk_delete): Stockpile management, remove.
