Would it make sense to add a parameter to _getfinalpathname that specifies the type of the path? For same[open]file(), we can probably just go to the most unreadable but broadly supported type, whereas the other functions that are actually going to return the path can work their way up through easiest-to-read options.

I like the proposed logic for st_dev and st_ino, though I don't see any value in testing the drive name separately. At that point we've already gone to the filesystem driver, so any chances of doing it fast are gone and we may as well have simpler logic that goes straight to the full path.