[$] Two pidfd tweaks: PIDFD_GET_INFO and PIDFD_SELF

The pidfd mechanism, which uses file descriptors to refer to processes in
an unambiguous and race-free way, was first
introduced 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 working
with pidfds simpler in some situations.