Remove constructors from FileEntry that prevent owning resources

This cleans up some constructors that would not be safe once FileEntry
owns the storage for its name. These were already suspect, since they
wouldn't work if the FileEntry had an open file descriptor. The only
user for these constructors was in UniqueFileContainer, which wasn't a
very useful abstraction anyway. So it and UniqueDirContainer have been
replaced with std::map<UniqueID, *>.

This change should not affect anything outside the FileManager.