If `fd_readdir` returns a zero inode, call `fstatat` to get the inode value. (#345)

If `fd_readdir` returns a zero inode, call `fstatat` to get the inode value. (#345)

* If `fd_readdir` returns a zero inode, call `fstatat` to get the inode value.

On some systems, `fd_readdir` may not implement the `d_ino` field and
may set it to zero. When this happens, have wasi-libc call `fstatat` to
get the inode number.

See the discussion in
https://github.com/WebAssembly/wasi-filesystem/issues/65 for details.

* Update the `d_type` field too, in case it changes.