[$] Adding the pidfd abstraction to the kernel

One of the many changes in the 5.4 kernel is the completion (insofar asanything in the kernel is truly complete) of the pidfd API. Getting that work done has been "awild ride so far", according to its author Christian Brauner during asession at the 2019 Kernel Recipes conference. He went on to describethe history of this work and some lessons for others interested in addingmajor new APIs to the Linux kernel.