Support for network mapped drives on Windows

[jannistpl]

Thanks for this awesome crate! I really appreciate the work :)

I'm currently using the sysinfo crate to get the available system disks in a file dialog. This works fine, but on Windows the Disks currently do not include mapped drives (e.g. mapped network drives). Although these are not technically considered "disks", I would expect a sysinfo crate to provide a list of all available volumes/filesystems/drives on the system.

Although I could write a very simple implementation using the Windows API myself for my project, I think it would be better if a crate like sysinfo can provide this information.