afp: Use UTF-8 for volume names

Use UTF-8 for volume names since volume names are encoded as UTF-8 for
protocol versions 3.0 and higher.  This prevents seeing volume names
like: "John‚Äôs Public Folder"

Since the volume name comes as a pascal string with UTF-8 encoding,
extend the function to read pascal strings with the ability to read
pascal strings encoded as UTF-8.

https://bugzilla.gnome.org/show_bug.cgi?id=733996