Default, vessels: Darken shelf empty slot icons

Rename slot icons.

mods/default/README.txt

@@ -141,7 +141,7 @@ BlockMen (CC BY-SA 3.0):
   gui_*.png
 
 Wuzzy (CC BY-SA 3.0):
-  default_book_slot.png (based on default_book.png)
+  default_bookshelf_slot.png (based on default_book.png)
 
 sofar (CC BY-SA 3.0):
   default_book_written.png, based on default_book.png

mods/default/nodes.lua

@@ -1817,7 +1817,7 @@ for i = 1, 16 do
 		by = by + 1
 	end
 	bookshelf_formspec = bookshelf_formspec ..
-		"image["..bx..","..by..";1,1;default_book_slot.png]"
+		"image[" .. bx .. "," .. by .. ";1,1;default_bookshelf_slot.png]"
 	bx = bx + 1
 end
 

mods/vessels/README.txt

@@ -19,4 +19,4 @@ The following textures were modified by Thomas-S (CC BY-SA 3.0):
   vessels_steel_bottle.png
 
 The following texture was created by Wuzzy (CC BY-SA 3.0):
-  vessels_glass_bottle_slot.png (based on vessels_glass_bottle.png)
+  vessels_shelf_slot.png (based on vessels_glass_bottle.png)

mods/vessels/init.lua

@@ -21,7 +21,7 @@ for i = 1,16 do
 		vy = vy + 1
 	end
 	vessels_shelf_formspec = vessels_shelf_formspec ..
-		"image["..vx..","..vy..";1,1;vessels_glass_bottle_slot.png]"
+		"image[" .. vx .. "," .. vy .. ";1,1;vessels_shelf_slot.png]"
 	vx = vx + 1
 end