Limit nukes to strategic missions

Limit nukes to strategic missions

Before Empire 2, nukes could be delivered only with bomb (special
mission 'n', airburst only) and launch (targeting sectors or
satellites only).

Empire 2 made nukes available for any kind of bombing, and for any
missile strike on sectors or ships.  This included interdiction and
support missions.  Nuclear-tipped anti-sats and bomb mission n were
removed.

Unfortunately, this was done in a messy way, which led to
inconsistencies and bugs.  The problem is that ordinary damage affects
just the target, while nuke damage affects an area.  Code dealing with
plane damage was designed for the former.  Instead of rewriting it to
cope with area damage cleanly, nuke damage got shoehorned into
pln_damage(), the function to compute conventional plane damage, as a
side effect: computing damage blasted sectors in the area.

If the plane carried a nuke, pln_damage() returned zero (conventional)
damage.  Without further logic, this simply bypassed the code to apply
damage to the target.  This worked out okay when the target already
got damaged correctly by the side effect.

However, some targets are immune to the side effect: when interdicting
a move or explore command, the commodities being moved are not in any
sector.

For other targets, damage has effects other than damaging the target:
offensive and defensive support don't apply the (conventional) damage
to the target sector.  Instead, they turn it into a combat bonus.
Without further logic, nuclear damage doesn't contribute to that.

To make all that work, pln_damage() returned the nuclear damage for
ground zero as well.  Because a plane does either conventional or
nuclear damage, one of them is always zero.

Most callers simply ignored the nuclear damage, and applied only the
conventional damage.

Bug: land units and ships failed to retreat when pin-bombed or
missiled with a nuke.  That's because they received zero conventional
damage.

The mission code flies planes and missiles and tallies their damage.
This mission damage included nuclear damage at ground zero (except for
missiles sometimes, see below), to make support and commodity
interdiction work.  Unfortunately, this broke other things.

Bug: when bombers interdicted ships or land units, nukes first damaged
the ships or land units by the side effect, then again through mission
damage.  Interdicting missiles had a special case to avoid this.

Bug: when interdicting move, explore or transport, nukes first damaged
the sector by the side effect, then again through mission damage's
collateral damage.

There may well be more bugs hiding in this mess.

The mess is not worth fixing.  While the idea of interdicting and
supporting with nukes sounds kind of cool, I believe it's pretty
irrelevant in actual play.

Instead, go back to a variation of the original rules: nukes can be
delivered only through bomb mission 's' and launch at sectors.

Make arm reject marine missiles in addition to satellites, ABMs and
SAMs, and clear the mission.  Make mission reject planes armed with
nukes.  Oops when they show up in mission_pln_equip() anyway.

Make pln_equip() allow planes with nukes only for missions 's' and
't'.

Clean up pln_damage() to just compute damage, without side effect.
Change strat_bomb() and launch_missile() to detonate nukes.  Simplify
the other callers.  Parameter mission of msl_launch_mindam() is now
unused, remove it.

Missiles exploding on launch no longer set off their nukes.  That was
pretty ridiculous anyway.