Avoid __pairs metamethod in stdnse.keys

This allows stdnse.keys to be used in a __pairs metamethod to, for
instance, yield keys in sorted order. Using next() bypasses the __pairs
metamethod that would be called when pairs() was used. Otherwise,
infinite recursion was possible.

nselib/stdnse.lua

@@ -1416,8 +1416,10 @@ end
 -- @return A table of keys
 function keys(t)
   local ret = {}
-  for k, _ in pairs(t) do
+  local k, v = next(t)
+  while k do
     ret[#ret+1] = k
+    k, v = next(t, k)
   end
   return ret
 end