[cs] readDirectory bad encoding

[Aurel300]

Put files whose names contain U+1FFFF, U+FFFFF, or U+10FFFF into a directory:

$ mkdir somedir
$ touch somedir/`printf "\xf0\x9f\xbf\xbf"` # U+1FFFF
$ touch somedir/`printf "\xf3\xbf\xbf\xbf"` # U+FFFFF
$ touch somedir/`printf "\xf4\x8f\xbf\xbf"` # U+10FFFF
$ ls somedir | xxd
0000000: f09f bfbf 0af3 bfbf bf0a f48f bfbf 0a    ...............

Read the directory with Haxe / C#:

class CsReadDirectory {
  public static function main():Void {
    trace(sys.FileSystem.readDirectory("somedir"));
  }
}

$ haxe --main CsReadDirectory --cs out.cs
$ mono out.cs/bin/CsReadDirectory.exe
FindNextFile: Bad encoding for 'somedir/�'
Consider using MONO_EXTERNAL_ENCODINGS

FindNextFile: Bad encoding for 'somedir/�'
Consider using MONO_EXTERNAL_ENCODINGS

FindNextFile: Bad encoding for 'somedir/�'
Consider using MONO_EXTERNAL_ENCODINGS

CsReadDirectory.hx:3: []

Expected output:

$ haxe --main CsReadDirectory --interp
CsReadDirectory.hx:3: [�,�,�]
$ haxe --main CsReadDirectory --interp | xxd
0000000: 4373 5265 6164 4469 7265 6374 6f72 792e  CsReadDirectory.
0000010: 6878 3a33 3a20 5bf0 9fbf bf2c f3bf bfbf  hx:3: [....,....
0000020: 2cf4 8fbf bf5d 0a                        ,....].

Tested on HFS+.

Once fixed:

• re-enable tests marked with this issue number in tests/sys/src/TestUnicode.hx