Add support for MINETEST_USERDATA environment variable (#12639)

The MINETEST_USER_PATH environment variable can be used to define a
custom path for Minetest user data. If MINETEST_USER_PATH is empty or
unset, the HOME (or APPDATA on Windows) environment variable is used as
the default user data path; this ensures backwards compatibility with
existing user setups.

src/main.cpp

@@ -527,7 +527,7 @@ static bool create_userdata_path()
 	}
 #else
 	// Create user data directory
-	success = fs::CreateDir(porting::path_user);
+	success = fs::CreateAllDirs(porting::path_user);
 #endif
 
 	return success;