fix map() command on 32-bit platforms; regressed in 5.2.0

fix map() command on 32-bit platforms; regressed in 5.2.0

LuaJIT FFI was using opendir() (etc.) variants with 32-bit inodes
but the C parts was using them as 64-bit inode variants.
Consequently the `struct dirent` layout didn't match and we were getting
filenames shifted by eight bytes.

Now the whole dir-listing lua function is written in C.