POSIX has recvfrom(2) and sendto(2), but their name seem not to be
suitable with Rust. We already renamed getpeername(2) and
getsockname(2), so I think it makes sense.
Alternatively, `receive_from` would be fine. However, we have `.recv()`
so I chose `recv_from`.
Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>