Make large military locations globally unique

Unlike faction bases, these locations aren't 'special' - they just add
location variety, and so the issue of making the player spawn location
special, and the chance of missing them if you go the wrong direction
are much less relevant. They _should_ be rare, so if you miss them, it's
not a problem.

Making the spawn location special is not great, but I think is
acceptable for this.

data/json/overmap/overmap_special/aircraft_carrier.json

@@ -201,6 +201,6 @@
     "city_distance": [ -1, 1000 ],
     "locations": [ "lake_surface" ],
     "occurrences": [ 75, 100 ],
-    "flags": [ "LAKE", "MAN_MADE", "UNIQUE", "MILITARY" ]
+    "flags": [ "LAKE", "MAN_MADE", "GLOBALLY_UNIQUE", "MILITARY" ]
   }
 ]

data/json/overmap/overmap_special/specials.json

@@ -7398,7 +7398,7 @@
     "city_sizes": [ 4, -1 ],
     "occurrences": [ 45, 100 ],
     "//": "Inflated chance, in effect ~10%",
-    "flags": [ "MILITARY", "UNIQUE", "MAN_MADE" ]
+    "flags": [ "MILITARY", "GLOBALLY_UNIQUE", "MAN_MADE" ]
   },
   {
     "type": "overmap_special",