Integrate air defense missions into interception

The ancients designed interception dead simple: when you overfly a
sector, you get intercepted by the sector owner.  Fine print
interception rules govern which planes intercept.

Then complexity got piled on top of it.

Chainsaw 2 added an extra interception by surface ship owners, in the
target sector only.

Chainsaw 3 added an extra interception by land unit owners, in the
target sector only (Empire 4 later merged this extra land unit
interception with the extra surface ship interception).

Chainsaw 3 added an entirely separate kind of interception: air
defense missions.  When you enter a sector in some air defense op
area, you get intercepted.  Fine print air defense rules govern which
planes intercept.  These rules differ significantly from the
interception fine print.

Additional complexity comes from these facts:

* Air defense mission interception happens in addition to non-mission
  interception.  You can boost your total interception by setting up
  air defense.  Which means you must set it up, or forgo an advantage.

* Air defense planes are not available for non-mission interception
  duty.  You need to decide on a split.

* In contrast to non-mission interception, interceptors flying air
  defense get intercepted.

Moreover, the air defense code breaks one of the plane code's design
assumptions, namely that just one plane sortie is active at a time.
The air defense sortie runs while the sortie it intercepts is in
progress.  This leads to two interceptions being active at the same
time: the one intercepting the original sortie, and the one
intercepting the air defense sortie.  The same plane can fly in both
interceptions, and damage received in the interception of the air
defense sortie is wiped out, triggering a seqno mismatch oops.

The previous commit already simplified non-mission interception: you
get intercepted by anyone who owns the sector, or a surface ship or a
land unit there, whether it's the target sector or not.

Now simplify mission interception, by merging air defense back into
ordinary interception: when you overfly a sector, you get intercepted
by anyone who owns the sector, or a surface ship or land unit there,
or has an air defense mission covering the sector.  That's all.  No
multiple interceptions, no separate air defense rules.

Remove air_defense().  Simplify ac_encounter() and sam_intercept()
accordingly; both lose their last parameter.

Change sam_intercept() and ac_intercept() to intercept in mission op
areas.  New parameter only_mission to suppress non-mission
interception.  Pass zero when the intercepting country owns the sector
or a surface ship or land unit in the sector.

ac_encounter() can't efficiently predict whether a country intercepts,
so it needs to call ac_intercept() unconditionally.  This kills the
optimization to collect interceptors only when needed; simplify
accordingly, replacing getilist() by getilists().

src/lib/commands/bomb.c

@@ -140,7 +140,7 @@ bomb(void)
 	return RET_FAIL;
     }
     pln_arm(&esc_list, 2 * ap_to_target, mission, ip, P_F | P_ESC);
-    ac_encounter(&bomb_list, &esc_list, ax, ay, flightpath, 0, 0);
+    ac_encounter(&bomb_list, &esc_list, ax, ay, flightpath, 0);
     if (QEMPTY(&bomb_list)) {
 	pr("No planes got through fighter defenses\n");
     } else if (target.sct_type == SCT_SANCT) {