[$] Grabbing file descriptors with pidfd_getfd()

In response to a growing desire for ways to control groups of processesfrom user space, the kernel has added a number of mechanisms that allow oneprocess to operate on another. One piece that is currently missing,though, is the ability for a process to snatch a copy of an open filedescriptor from another. That gap may soon be filled, though, if the pidfd_getfd()system-call patch set from Sargun Dhillon is merged.