[$] Special file descriptors in BPF

Developers learning the Unix (or POSIX in general) system-call set willquickly encounter file descriptors, which are used to represent open filesand more. Developers also tend to learn early on that the first three filedescriptors are special, with file descriptor zero being the standard inputstream, one being standard output, and two being standard error. Thekernel, though, does not normally attach any specific meaning to a givendescriptor number, so it was somewhat surprising when a recent BPF patchseries attempted to attach a special meaning to zero when used as a filedescriptor.