Get rid of struct plnstr member pln_nuktype

Get rid of struct plnstr member pln_nuktype

pln_nuktype is redundant; it can be computed from the nuke's
nuk_plane.

Make plane selector nuketype virtual and NSC_EXTRA.  It should have
been NSC_EXTRA all along.  This changes xdump plane.

Don't set it in arm(), disarm(), build_plane(), pln_damage() and
nuk_fixup().  The latter no longer does anything, remove it.

Deprecate edit key 'n' in doplane(), and don't show it in pr_plane().
The key never made much sense.

eff_bomb(), comm_bomb(), ship_bomb(), plane_bomb(), land_bomb(),
strat_bomb(), mission_pln_equip(), air_damage(), msl_hit(),
pln_equip() tested pln_nuketype to check whether a plane carries a
nuke.  Test nuk_on_plane() instead.

pdump(), plan(), trade_desc() print whether and what kind of nuke a
plane carries.  Adapt that to use nuk_on_plane().