For datagram unix sockets, the `SO_SNDBUF` value imposes an upper limit on the size of outgoing datagrams. Note: We don't support to change the buffer size yet, so only add support to getsockopt now. Refs: https://man7.org/linux/man-pages/man7/unix.7.html Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>