Add a mode to use a dummy Wii NAND.

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.

Source/Core/Common/FileUtil.h

@@ -20,8 +20,8 @@
 enum {
 	D_USER_IDX,
 	D_GCUSER_IDX,
-	D_WIIROOT_IDX,
-	D_WIIUSER_IDX,
+	D_WIIROOT_IDX, // always points to User/Wii or global user-configured directory
+	D_SESSION_WIIROOT_IDX, // may point to minimal temporary directory for determinism
 	D_CONFIG_IDX, // global settings
 	D_GAMESETTINGS_IDX, // user-specified settings which override both the global and the default settings (per game)
 	D_MAPS_IDX,
@@ -39,14 +39,11 @@ enum {
 	D_LOAD_IDX,
 	D_LOGS_IDX,
 	D_MAILLOGS_IDX,
-	D_WIISYSCONF_IDX,
-	D_WIIWC24_IDX,
 	D_THEMES_IDX,
 	F_DOLPHINCONFIG_IDX,
 	F_DEBUGGERCONFIG_IDX,
 	F_LOGGERCONFIG_IDX,
 	F_MAINLOG_IDX,
-	F_WIISYSCONF_IDX,
 	F_RAMDUMP_IDX,
 	F_ARAMDUMP_IDX,
 	F_FAKEVMEMDUMP_IDX,
@@ -122,6 +119,9 @@ void CopyDir(const std::string &source_path, const std::string &dest_path);
 // Set the current directory to given directory
 bool SetCurrentDir(const std::string &directory);
 
+// Creates and returns the path to a new temporary directory.
+std::string CreateTempDir();
+
 // Get a filename that can hopefully be atomically renamed to the given path.
 std::string GetTempFilenameForAtomicWrite(const std::string &path);