Replace mem::zeroed with mem::MaybeUninit::uninit for large struct in Unix As discussion in #136737. - Replace `mem::zeroed()` with `MaybeUninit::uninit()` for `sockaddr_storage` in `accept()` and `recvfrom()` since these functions fill in the address structure - Replace `mem::zeroed()` with `MaybeUninit::uninit()` for `pthread_attr_t` in thread-related functions since `pthread_attr_init()` initializes the structure - Add references to man pages to document this behavior