Use Lua 5.4 __close value for lfs.dir() iterator

[hishamhm]

Adds support for the __close value in the lfs.dir() iterator when using Lua 5.4. This requires no changes from the user, and ensures that the directory will be closed correctly even if you break from the loop.

For example, with uname -n set below 10000, this script crashes on Lua < 5.4, and works in Lua 5.4:

local lfs = require("lfs")
for t = 1, 10000 do
   local i = 0
   for e in lfs.dir("/usr/lib") do
      i = i + 1
      if i == 100 then
         break
      end
   end
end

(This is not a forced example: I have hit this problem before when recursing over large directory trees and collecting files.)

[coveralls]

Coverage decreased (-0.3%) to 81.922% when pulling 88044c6 on 5.4-iterator into 4f7e1b5 on master.