remove the optional subdir parameter of Get{Data,User}Dir

It's mostly unused; it's much less important with separate FileSource
objects for game data files and user files, since the FileSource objects
deal with knowing the base path (and other things).

src/FileSystem.cpp

@@ -14,7 +14,7 @@
 namespace FileSystem {
 
 	static FileSourceFS dataFilesApp(GetDataDir());
-	static FileSourceFS dataFilesUser(GetUserDir("data"));
+	static FileSourceFS dataFilesUser(JoinPath(GetUserDir(), "data"));
 	FileSourceUnion gameDataFiles;
 	FileSourceFS userFiles(GetUserDir());
 

src/FileSystem.h

@@ -35,8 +35,8 @@ namespace FileSystem {
 	extern FileSourceUnion gameDataFiles;
 	extern FileSourceFS userFiles;
 
-	std::string GetUserDir(const char *subdir = 0);
-	std::string GetDataDir(const char *subdir = 0);
+	std::string GetUserDir();
+	std::string GetDataDir();
 
 	/// Create path <a>/<b>, coping with 'a' or 'b' being empty,
 	/// 'b' being an absolute path, or 'a' not having a trailing separator

src/Pi.cpp

@@ -244,7 +244,7 @@ static void LuaInitGame() {
 
 std::string Pi::GetSaveDir()
 {
-	return FileSystem::GetUserDir("savefiles");
+	return FileSystem::JoinPath(FileSystem::GetUserDir(), "savefiles");
 }
 
 void Pi::Init()

src/posix/FileSystemPosix.cpp

@@ -77,22 +77,16 @@ namespace FileSystem {
 #endif
     }
 
-	std::string GetUserDir(const char *subdir)
+	std::string GetUserDir()
 	{
 		static const std::string user_path = FindUserDir();
-		if (subdir)
-			return JoinPathBelow(user_path, subdir);
-		else
-			return user_path;
+		return user_path;
 	}
 
-	std::string GetDataDir(const char *subdir)
+	std::string GetDataDir()
 	{
         static const std::string data_path = FindDataDir();
-		if (subdir)
-			return JoinPathBelow(data_path, subdir);
-		else
-			return data_path;
+		return data_path;
 	}
 
 	FileSourceFS::FileSourceFS(const std::string &root):

src/win32/FileSystemWin32.cpp

@@ -65,22 +65,16 @@ namespace FileSystem {
 		return transcode_utf16_to_utf8(buf, wcslen(buf));
 	}
 
-	std::string GetUserDir(const char *subdir)
+	std::string GetUserDir()
 	{
 		static const std::string user_path = FindUserDir();
-		if (subdir)
-			return JoinPathBelow(user_path, subdir);
-		else
-			return user_path;
+		return user_path;
 	}
 
-	std::string GetDataDir(const char *subdir)
+	std::string GetDataDir()
 	{
 		static const std::string data_path = FindDataDir();
-		if (subdir)
-			return JoinPathBelow(data_path, subdir);
-		else
-			return data_path;
+		return data_path;
 	}
 
 	FileSourceFS::FileSourceFS(const std::string &root):