Added keep and castle overlay terrains,

...which can be painted on any non-layered terrain to enable them to
act as keeps or castles for recruitment.

data/core/terrain.cfg

@@ -340,6 +340,29 @@
 #editor_group=castle
 #[/terrain]
 
+[terrain]
+    symbol_image=alpha
+    editor_image=keep-overlay-editor
+    id=keep_overlay
+    name= _ "Keep"
+    string=^Kov
+    aliasof=_bas
+    recruit_from=true
+    recruit_onto=true
+    editor_group=castle, special
+[/terrain]
+
+[terrain]
+    symbol_image=alpha
+    editor_image=castle-overlay-editor
+    id=castle_overlay
+    name= _ "Castle"
+    string=^Cov
+    aliasof=_bas
+    recruit_onto=true
+    editor_group=castle, special
+[/terrain]
+
 #
 # Desert
 #