Added minetest.register alias on mirror and fixed seawrecks' crash

- Created alias from returmirror:mirror_inactive to returnmirror:portal so the
  players will be able to find this item in unified_inventory's search menu by
  typing "portal"
- Fixed a very strange and unhandled crash in seawrecks, caused apparently by
  minetest.add_node. Used minetest.set_node instead and everything is fine

mods/returnmirror/init.lua

@@ -125,3 +125,5 @@ minetest.register_craft({
 		{"default:diamondblock", "default:nyancat", "default:diamondblock"},
 	},
 })
+
+minetest.register_alias("returnmirror:portal","returnmirror:mirror_inactive")

mods/sea/seawrecks/init.lua

@@ -950,7 +950,7 @@ local yp = {x = pos.x, y = pos.y + 8, z = pos.z}
 		pos.y = pos.y - 7
 		pos.x = pos.x +16
 		pos.z = pos.z +3
-		minetest.add_node(pos, {name = "seawrecks:ubootchest"})
+		minetest.set_node(pos, {name = "seawrecks:ubootchest"})
 
 	else
 		return