Searchlights: don't drop items used for settings (#63429)

Searchlights store their settings on a processor board item, so their
"beams" can have persistent behavior between turns. They do this using
item vars, but these item vars linger after their death, causing the
items not to stack.
Simple solution: Just don't drop these settings items.
Also, erase these item vars on any existing items, so that they can now
stack.

This does mean that searchlights are no longer guaranteed to drop
multiple processor items. I think that this is acceptable, but it will
mean they drop fewer items as, unlike other robots, they do not drop a
disassemblable corpse, only a random item from an itemgroup.

src/monattack.cpp

@@ -3366,6 +3366,9 @@ bool mattack::check_money_left( monster *z )
 
             if( !z->inv.empty() ) {
                 for( const item &it : z->inv ) {
+                    if( it.has_var( "DESTROY_ITEM_ON_MON_DEATH" ) ) {
+                        continue;
+                    }
                     get_map().add_item_or_charges( z->pos(), it );
                 }
                 z->inv.clear();
@@ -3809,6 +3812,7 @@ bool mattack::searchlight( monster *z )
 
             settings.set_var( "SL_SPOT_X", 0 );
             settings.set_var( "SL_SPOT_Y", 0 );
+            settings.set_var( "DESTROY_ITEM_ON_MON_DEATH", "TRUE" );
 
             z->add_item( settings );
         }

src/monexamine.cpp

@@ -196,6 +196,9 @@ void dump_items( monster &z )
     Character &player_character = get_player_character();
     map &here = get_map();
     for( item &it : z.inv ) {
+        if( it.has_var( "DESTROY_ITEM_ON_MON_DEATH" ) ) {
+            continue;
+        }
         here.add_item_or_charges( player_character.pos(), it );
     }
     z.inv.clear();

src/monster.cpp

@@ -2671,6 +2671,9 @@ void monster::die( Creature *nkiller )
     }
     if( death_drops && !is_hallucination() ) {
         for( const item &it : inv ) {
+            if( it.has_var( "DESTROY_ITEM_ON_MON_DEATH" ) ) {
+                continue;
+            }
             if( corpse ) {
                 corpse->force_insert_item( it, item_pocket::pocket_type::CONTAINER );
             } else {

src/savegame_json.cpp

@@ -3058,9 +3058,16 @@ void item::io( Archive &archive )
         }
     }
 
-    // Remove stored translated gerund in favor of storing the inscription tool type
-    item_vars.erase( "item_label_type" );
-    item_vars.erase( "item_note_type" );
+    static const std::set<std::string> removed_item_vars = {
+        // Searchlight monster setting vars
+        "SL_PREFER_UP", "SL_PREFER_DOWN", "SL_PREFER_RIGHT", "SL_PREFER_LEFT", "SL_SPOT_X", "SL_SPOT_Y", "SL_POWER", "SL_DIR",
+        // Remove stored translated gerund in favor of storing the inscription tool type
+        "item_label_type", "item_note_type"
+    };
+
+    for( const std::string &var : removed_item_vars ) {
+        item_vars.erase( var );
+    }
 
     current_phase = static_cast<phase_id>( cur_phase );
     // override phase if frozen, needed for legacy save