Replace mem::zeroed with mem::MaybeUninit::uninit for large struct in Unix

[xizheyin]

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