[mapgen] Fix old-style API calls, make the code runnable

mods/MAPGEN/mcl_strongholds/init.lua

@@ -66,8 +66,10 @@ local init_strongholds = function()
 	strongholds_inited = true
 end
 
+init_strongholds()
+
 -- Stronghold generation for register_on_generated.
-local generate_strongholds = function(minp, maxp, blockseed)
+mcl_mapgen.register_chunk_generator(function(minp, maxp, blockseed)
 	local pr = PseudoRandom(blockseed)
 	for s=1, #strongholds do
 		if not strongholds[s].generated then
@@ -99,8 +101,4 @@ local generate_strongholds = function(minp, maxp, blockseed)
 			end
 		end
 	end
-end
-
-init_strongholds()
-
-mcl_mapgen_core.register_generator("strongholds", nil, generate_strongholds, 999999)
+end, 999999)

mods/MAPGEN/mcl_villages/init.lua

@@ -78,7 +78,7 @@ end
 -- Disable natural generation in singlenode.
 local mg_name = minetest.get_mapgen_setting("mg_name")
 if mg_name ~= "singlenode" then
-	mcl_mapgen_core.register_generator("villages", nil, function(minp, maxp, blockseed)
+	mcl_mapgen.register_chunk_generator(function(minp, maxp, blockseed)
 		-- don't build settlement underground
 		if maxp.y < 0 then return end
 		-- randomly try to build settlements

mods/MAPGEN/tsm_railcorridors/init.lua

@@ -1089,7 +1089,7 @@ local function create_corridor_system(main_cave_coords)
 end
 
 -- The rail corridor algorithm starts here
-mcl_mapgen_core.register_generator("railcorridors", nil, function(minp, maxp, blockseed, _pr)
+mcl_mapgen.register_chunk_generator(function(minp, maxp, blockseed, _pr)
 	-- We re-init the randomizer for every mapchunk as we start generating in the middle of each mapchunk.
 	-- We can't use the mapgen seed as this would make the algorithm depending on the order the mapchunk generate.
 	InitRandomizer(blockseed)