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.
Change broke the build on Debian stable.
This reverts commit 28755439b36ac8e875d764da5db31ad693459ace, reversing
changes made to 64e01ec763d21915e7166e625f8a3e85b46fbcbb.
