[boost] [Filesystem] Add features to list all mount paths, determine mount path of a path

John Reynolds came up with a great idea for a possible extension to Boost Filesystem, in response to a question on the Boost Users' List about listing all available drives on Windows. The basic idea is to have the ability to list mounted filesystems. I then suggested that `basic_path` could be extended with a new member (e.g.: `mount_path`) that will determine the mount path of the given path, and add a function to iterate over all mount paths. Before I open a feature request, I wanted to ask some questions: 1. Has this idea been discussed before? 2. Is `mount_path` a good name for this? 3. What would be a good name for the function to iterate over all mount paths? 4. Is the concept of mount paths cross-platform?