Fix potential fd leak in flingfd_recv.

Although I declared enough space for a single file descriptor, the
CMSG_SPACE macro applies padding which can be used to store additional
descriptors. In other words, your own cmsg->cmsg_len can be greater
than what you'd intended, resulting in a descriptor leak. An adversary
can use that to DoS your process.

This change addresses the issue by closing all incoming fds if more
than one was sent per datagram.