lib-fs: Split allocation from fs.iter_init() to a separate fs.iter_alloc()

lib-fs: Added fs_get_nlinks() Although fs_stat() could return this, its caller can't indicate whether it actually wants the link count. Usually fs_stat() is used only to get the file's size. In some backends it's not cheap to get the link count, so adding this function allows the caller to explicitly ask for it.

lib-fs: Implement all lib-fs functions as wrappers to parent stream.