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.

Eventually, netplay will be able to use the host's NAND, but this could
still be useful in some cases; for TAS it definitely makes sense to have
a way to avoid using any preexisting NAND.

In terms of implementation: remove D_WIIUSER_IDX, which was just WIIROOT
+ "/", as well as some other indices which are pointless to have as
separate variables rather than just using the actual path (fixed, since
they're actual Wii NAND paths) at the call site.  Then split off
D_SESSION_WIIROOT_IDX, which can point to the dummy NAND directory, from
D_WIIROOT_IDX, which always points to the "real" one the user
configured.