Restore the new implementation of the backstab weapon special

This reverts commit e0ac69ce96.

data/core/macros/abilities.cfg

@@ -402,8 +402,16 @@ Enemy units cannot see this unit while it is in deep water, except if they have
         name= _ "backstab"
         description= _ "When used offensively, this attack deals double damage if there is an enemy of the target on the opposite side of the target, and that unit is not incapacitated (turned to stone or otherwise paralyzed)."
         multiply=2
-        backstab=yes
         active_on=offense
+        [filter_opponent]
+            [filter_adjacent]
+                adjacent=$other_unit.facing
+                is_enemy=yes
+                [not]
+                    status=petrified
+                [/not]
+            [/filter_adjacent]
+        [/filter_opponent]
     [/damage]
 #enddef
 

src/unit_abilities.cpp

@@ -1006,6 +1006,10 @@ effect::effect(const unit_ability_list& list, int def, bool backstab) :
 		const config& cfg = *ability.first;
 		std::string const &effect_id = cfg[cfg["id"].empty() ? "name" : "id"];
 
+		if (!cfg["backstab"].blank()) {
+			lg::wml_error << "The backstab= key in weapon specials is deprecated; use [filter_adjacent] instead\n";
+		}
+
 		if (!backstab && cfg["backstab"].to_bool())
 			continue;
 		if (!filter_base_matches(cfg, def))