Use rewinddir to count number of files

[Cong]

In tinydir_open_sorted, a first pass is used to count the number of files, then a second pass actually reads the file info. In between the two passes, tinydir closes and reopens the dir.

This is unnecessary with dirent; there's the rewinddir function which allows going back to the first file. This would also eliminate a race condition where the directory may have changed between the two passes.

Not sure if there's an equivalent for Windows.

[Lautis Sun]

What about malloc() a small block of memory (maybe size * 12, I guess), and realloc() another block when needed? Will need only one-pass. Might be slower, but safer about race condition.
There could be a balance between safety and efficiency.

[Cong]

I tried a doubling realloc implementation before but counting in two passes was faster on a folder with 10000 items. Anyway rewinddir should be both fast and safe. See #6