Since all FS access will go through the new FS interface (PR #6421)
in order to keep track of metadata properly, there is no need to return
absolute paths anymore.
In fact, returning host paths is a roadblock to using the FS interface.
This starts the migration work by adding a way to get paths that are
relative to the Wii NAND instead of always getting absolute paths
on the host FS.
To prepare for future changes, this commit also makes returned paths
canonical by removing the trailing slash when it's unneeded.
Eventually, once everything has been migrated to the new interface,
we can remove the "from" parameter.
Léo Lam