[$] Two pidfd tweaks: PIDFD_GET_INFO and PIDFD_SELF

The pidfd mechanism, which uses file descriptors to refer to processes inan unambiguous and race-free way, was firstintroduced in 2018. Since then, the interface has gained a number of new features, but development has slowed over time as the interface has matured. There are,however, a couple of patches in circulation that are meant to make workingwith pidfds simpler in some situations.