Default: Add metal sound table function and metal sounds

mods/default/README.txt

@@ -231,3 +231,14 @@ Mito551 (sounds) (CC BY-SA 3.0):
   default_dirt_footstep.1.ogg
   default_dirt_footstep.2.ogg
   default_glass_footstep.ogg
+
+Metal sounds:
+  default_dig_metal.ogg - yadronoff - CC-BY-3.0
+  - https://www.freesound.org/people/yadronoff/sounds/320397/
+  default_dug_metal.*.ogg - Iwan Gabovitch - qubodup - CC0
+  - http://opengameart.org/users/qubodup
+  default_metal_footstep.*.ogg - Ottomaani138 - CC0
+  - https://www.freesound.org/people/Ottomaani138/sounds/232692/
+  default_place_node_metal.*.ogg - Ogrebane - CC0
+  - http://opengameart.org/content/wood-and-metal-sound-effects-volume-2
+

mods/default/functions.lua

@@ -95,6 +95,19 @@ function default.node_sound_glass_defaults(table)
 	return table
 end
 
+function default.node_sound_metal_defaults(table)
+	table = table or {}
+	table.footstep = table.footstep or
+			{name = "default_metal_footstep", gain = 0.5}
+	table.dig = table.dig or
+			{name = "default_dig_metal", gain = 0.5}
+	table.dug = table.dug or
+			{name = "default_dug_metal", gain = 0.5}
+	table.place = table.place or
+			{name = "default_place_node_metal", gain = 0.5}
+	default.node_sound_defaults(table)
+	return table
+end
 
 --
 -- Lavacooling