An unsigned char fix and an errno change

[Nebuleon]

Here are 3 commits that improve the standards compliance of libfat in some specific scenarios:

• compiling _FAT_directory_lfnLength with char equivalent to signed char;
• setting the correct value of errno when removing a non-empty directory;
• not setting errno at the end of a directory in _FAT_dirnext if no error occurred while reading.

[FrankHB]

See also https://github.com/devkitPro/libfat/issues/10.