It's used by both the GUI to do things like install WADs and check up on
the system menu, in which case the global root should be used, and by
/dev/es, in which case the local one should. The latter isn't
*terribly* useful today, since no contents will ever be installed in
temporary roots (although it's still relevant for data directories), but
converting the whole thing makes sense because then it will Just Work
once the entire NAND is synced.
Because it would have been a bit of work to split it up (but I can if
desired), this commit also contains some basic cleanup of
NANDContentLoader:
(1) The useless interface class INANDContentLoader is removed and the
methods are changed to just return CNANDContentLoader (the only
implementation);
(2) CNANDContentManager is changed to use unique_ptr and cleaned up a
bit.
