When I do "strace shutdown -P now", I see:
recvmsg(4, {msg_name(0)=NULL, msg_iov(1)=[{"l\4\1\1!\2\0\0007\0\0\0\236\0\0\0\1\1o\0008\0\0\0", 24}], msg_controllen=32, [{cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS, {pid=1, uid=0, gid=0}}], msg_flags=MSG_CMSG_CLOEXEC}...