folder_file: remove struct stat

folder_file: remove struct stat

Add and use new flag to indicate the folder is on local filesystem.
Add and use new gid, uid and nlink member.
Use existing ->mode member instead of stat->st_mode.
Use existing ->size member instead of stat->st_size.
Use existing ->mtime member instead of stat->st_mtime.
Remove struct stat, the used values were already duplicated in the struct.
This reduces memory usage.