[$] Fending off unwanted file descriptors

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