[$] Fending off unwanted file descriptors

One of the more obscure features provided by Unix-domain sockets is the
ability to pass a file descriptor from one process to another. This
feature is often used to provide access to a specific file or network
connection to a process running in a relatively unprivileged context. But
what if the recipient doesn't want a new file descriptor? A feature
added for the 6.16 release makes it possible to refuse that offer.