Detecting attrib's /L option and support clearing symlinks' attributes

[Explorer09]

This is part of usb_vaccine.cmd to-do list.

Currently usb_vaccine.cmd ignores symlinks during the [attrib] step because there's not yet a way to detect if user's attrib utility supports "/L" option ("/L" is supported since Windows Vista). This brings a further technical limitation that we cannot move symlinks with Hidden or System attribute but have to delete them instead. usb_vaccine.cmd has been trying to avoid deleting any file from user's system, and ideally we can avoid deleting symlinks too, at least in Windows Vista or later.