Io: Provide directory existence with listing.

Sometimes, you need to tell the difference between an empty directory and
one that doesn't exist at all.  We can do this in a single call.

Core/FileSystems/BlobFileSystem.cpp

@@ -30,9 +30,11 @@ void BlobFileSystem::DoState(PointerWrap &p) {
 	// Not used in real emulation.
 }
 
-std::vector<PSPFileInfo> BlobFileSystem::GetDirListing(std::string path) {
+std::vector<PSPFileInfo> BlobFileSystem::GetDirListing(const std::string &path, bool *exists) {
 	std::vector<PSPFileInfo> listing;
 	listing.push_back(GetFileInfo(alias_));
+	if (exists)
+		*exists = true;
 	return listing;
 }