fixes bad rotation that was introduced with fd1e5a516a
(because nodeboxes can't be wallmounted, and facedir != wallmounted param2)
requires minetest from git commit dd4c21c1808acedfbcf8402c09ce9129b6ac31c7 or later.
package the two together as a modpack so that regular homedecor users don't
see a functional change, but others can take signs_lib and use it as a
standalone mod.
also get rid of some *~ files
