Use -fileSystemRepresentation instead of -UTF8String

[DustinNorman]

Updated fusefm_readdir to use -[NSString fileSystemRepresentation] to convert to C-strings. Using -[NSString UTF8String] can cause issues with duplicated folder/file names in Finder if the name contains accented characters due to Unicode normalization.

I believe this to resolve issue: https://github.com/osxfuse/osxfuse/issues/460. I was seeing the same behavior and this change resolves it.