Rudimentary version of Wii IPC determinism.  Ported from my old udpnet branch.

I forgot it when I added the method.

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.

Use an ID-to-name map when volume has no company string

This is written so that the result of GetCompanyFromID never is cached
(except on Android?). Caching is unnecessary because the string can be
obtained quickly at runtime, and not caching it means that the cache
doesn't have to be invalidated when GetCompanyFromID is edited.

This makes Dolphin display the same names as the Disc Channel.

Having some data available in banner loaders and some other data
data available in volumes gets messy, especially with GetNames(),
which is available in both but returns different results
depending on which one is used. This change drops support
for reading names and descriptions from Wii save data.

This makes the code cleaner and also leads to some user-visible changes:

The wx game properties will no longer let the user
select WAD languages that don't have any names.

The Qt game list will now display names using the languages
set in the configuration instead of always using
English for PAL GC games and Japanese for WADs.

If a WAD doesn't have a name in the user's preferred language,
English is now selected as a fallback before Japanese.